Studies were identified by searching PubMed for articles relating to our questions. Search results were restricted to the English language and peer-reviewed studies, with no restriction on year of publication. The search was last executed on November 20, 2019 in order to cover recent publications. Search queries were generated using the following combination of keywords: [“delirium” AND “noise OR sound OR light OR circadian”]. The search was applied with no field tags to maximize results.

After compiling research results and removing duplicates, HH and JL screened titles and abstracts to retrieve articles for eligibility. Articles on pediatric populations, animal subjects, case reports, or where the full-text was unavailable were excluded. Additional studies were identified through hand-searches and searching the reference list of reviewed articles. Disagreements on study eligibility were resolved by involving a third reviewer and a discussion between the reviewers. HH, JC, and JL reviewed the full text of eligible articles and extracted data using a pre-designed worksheet reviewed and tested by the team before data charting ( Table 1). Elements of the data charting worksheet included study design, setting, sample size, aim, detailed methodology, characteristics of intervention and control groups, measured outcomes, diagnostic tools, main conclusions, and study strengths and limitations. Disagreements were resolved by discussion.

We included observational studies analyzing the association between noise levels, light exposure or disrupted circadian rhythm and delirium, and interventional studies assessing the effectiveness of modified noise or light exposure or improved circadian rhythm on delirium. Articles were excluded if environmental intervention was an element of a multi-component non-pharmacological bundle, not the main focus. In the initial full text review and data charting, we reviewed all interventional articles reporting results on delirium or the environmental risk factors of delirium, including noise or light levels, and quality/quantity of sleep. We acknowledge these outcomes are modifiable risk factors linked to delirium prevention or management; however, we excluded articles without results linked to delirium.

The electronic database search retrieved 457 articles, which were screened by title and abstract, resulting in 166 studies for full-text review. Hand-search and the searching of reference lists added 28 additional articles. During the full-text review of these 194 articles, 157 were excluded. In total 37 studies were included: 21 assessed association between environmental risk factors and delirium ^{13, 19, 20, 30– 47} , and 16 reported on delirium after an environmental intervention ^{7, 14, 15, 18, 48– 59} ( Figure 1).

Included studies were conducted between 1997 to 2019, in the USA ^{19, 31, 37, 39, 43, 46, 58, 59} , the Netherlands ^{14, 32, 48, 50, 51} , Japan ^{38, 41, 52, 53} , France ^{33, 36, 57} , Belgium ^{13, 49} , Denmark ^{15, 34} , Italy ^{42, 47} , Sweden ^{18, 20} , Canada ³⁵ , China ⁴⁵ , India ⁴⁰ , Israel ³⁰ , Singapore ⁵⁵ , South Korea ⁵⁴ , Thailand ⁵⁶ , Turkey ⁴⁴ , and the UK ⁷ . Of these, 31 studies were conducted among critically ill patients while five reviewed general hospital populations ^{30, 35, 41, 44, 54} , and one a geriatric monitoring unit for acute delirium care ⁵⁵ . Among the 37 reviewed articles, all observational association studies and 12 interventional studies reported delirium incidence, while two interventional studies measured delirium prevalence ^{18, 58} . Delirium severity was assessed in three interventional studies ^{48, 54, 55} . Three articles reviewed delirium duration ^{7, 48, 51} .

Most studies assessed delirium using the Confusion Assessment Method for the ICU (CAM-ICU) ^{7, 15, 18, 19, 34, 36, 37, 39, 40, 42, 45– 48, 51, 56– 59} ; other identification methods included the validated Dutch CAM-ICU ³² , Confusion Assessment Method (CAM) ^{41, 50, 55} , Intensive Care Delirium Screening Checklist (ICDSC) ^{14, 33, 43} , Neelon and Champagne Confusion Scale (NEECHAM) ^{13, 49} , non-validated ⁵² and validated ⁵³ Japanese NEECHAM, Delirium Observation Screening Scale (DOSS) ⁵⁰ , behavioral observations based on the Diagnostic and Statistical Manual of Mental Disorders, 3 ^rd edition (DSM-III) ³⁵ , 3 ^rd edition-revised (DSM-III-R) ³⁸ , and 4 ^th Edition (DSM-IV) ^{20, 41, 44} , and behavioral observations based on International Classification of Diseases, 9 ^th Revision, Clinical Modification (ICD-9-CM) criteria ³⁰ . One study used both retrospective chart review and site-specific pre-specified criteria based on new and rapid onset of disturbed consciousness and/or perceptual disturbances ³¹ . Studies assessed delirium severity by non-validated Delirium Severity Index (DSI) ⁴⁸ , Delirium Rating Scale (DRS) ⁵⁴ , Delirium Rating Scale-Revised-98 (DRS-R-98) ⁵⁵ , and Memorial Delirium Assessment Scale (MDAS) ⁵⁴ . Details, including study design, setting, sample size, methodology, outcomes, and findings with statistics are summarized in Table 2 to Table 4 for observational studies reporting on environmental risk factors, and Table 5 to Table 7 for environmental intervention studies.

Of the observational studies, two analyzed for an association between delirium and noise ^{19, 20} , five for light ^{13, 30– 33} , 12 for sleep ^{34– 45} , and two evaluated multiple factors ^{46, 47} . Study populations ranged from 7 to 6660 participants, and the majority were done in an ICU (17 of 21) ^{13, 19, 31– 34, 36– 40, 42, 43, 45– 47} . The remaining studies did not specify a ward ^{30, 35, 41, 44} . Study details and statistical results are in Table 2– Table 4.

Noise. Although ICU noise is a suggested predictor for delirium development, two of the three investigating studies found no significant association between ICU noise levels and delirium development ^{19, 20} ( Table 2). One study assessed A-weighted sound levels with subjective patient reports on ICU noise ²⁰ . They found no correlation between A-weighted equivalent continuous (LAeq) or maximum (LAmax) noise pressure levels and delirium, while patients’ responses about ICU sounds spread evenly over a spectrum from scary to non-disturbing ²⁰ . In comparison, Knauert et al. ¹⁹ evaluated equivalent continuous sound pressure level (Leq) and peak sound occurrences for both A-weighted and C-weighted measurements, finding no correlation with delirium development. There are no industry-standard recommendations for C-weighted levels, but LAeq and LAmax values from both studies were higher than recommended by the WHO ^{17, 19, 20} . In contrast, a study by Davoudi et al. ⁴⁶ assessing the associations between delirium and multiple environmental factors, found average night-time sound pressure levels were significantly higher for patients with delirium ⁴⁶ . However, they did not provide exact decibel measurements, likely because they were reporting preliminary findings for a larger cohort study unpublished at the time of this review ⁴⁶ .

Light. Abnormal lighting cycles are another suggested contributor to delirium ⁶⁰ . Seven reviewed studies considered exposure to natural sunlight and relationships with delirium ^{13, 30– 33, 46, 47} ( Table 3). There were two approaches to analysis: effects of windows on delirium incidence ^{13, 31 , 33, 46, 47} and association with admission season ^{46, 47} . Findings were mixed, suggesting no easily provable relationship between natural light exposure and delirium occurrence. Two window and one seasonal study found no statistical association between delirium and windows or season of admission/duration of preadmission sunlight exposure, respectively ^{31– 33} . Kohn et al. ³¹ compared windowed versus non-windowed rooms in the medical ICU, and natural versus industrial window views in the surgical ICU. ³¹ . They also investigated impact of half-sized versus full-sized windows, finding no association between delirium incidence and any of these factors ³¹ . Similarly, Smonig et al. found no difference in delirium incidence between patients admitted to windowed versus non-windowed rooms while proving windowed rooms retained natural circadian light variations and non-windowed rooms did not ³³ . In the seasonal study, Simons et al. investigated the effect of admission season on delirium incidence and found no correlation ³² . A simultaneous assessment found no correlation between preadmission cumulative sunlight exposure and delirium incidence for three photoperiods (7, 28, and 60 days pre-hospital admission) ³² .

In comparison to studies showing no association between natural sunlight exposure and delirium occurrence, three window studies and one seasonal study found a significant correlation ^{13, 30, 46, 47} . In the window studies, Simeone et al. ⁴⁷ associated the lack of natural sunlight with delirium while Van Rompaey et al. ¹³ found an absence of visible daylight led to higher risk of delirium. Davoudi et al. ⁴⁶ examined the pervasive sensing of ICU patients, finding that the measured light intensity in windowed rooms was significantly different between patients with and without delirium ⁴⁶ . Additionally, a study on seasonal impact on delirium diagnosis by Balan et al. found a higher incidence of delirium among patients admitted in winter compared to summer ³⁰ .

Sleep. Disrupted sleep-wake cycles are associated with altered mental states in hospitalized patients, and are connected with delirium ⁶¹ . In this review, 14 studies ^{19, 34– 38, 40– 47} assessed sleep and delirium with two main methodologies: objective measurements of physiological sleep phases and subjective reports by staff or patient. Five studies objectively measured sleep quality using overnight polysomnography (PSG) or a Zeo wireless sleep monitor ^{19, 34, 36, 42, 43} , while eight assessed staff reports of behavioral observations and/or self-reports by patients ^{34, 35, 37, 38, 41, 45– 47} . One study compared both methods ³³ , and two did not specify the measurement method. ^{40, 44} ( Table 4).

Similar to the articles on natural light exposure, association studies for sleep and delirium have mixed findings, but lean towards disrupted sleep being a delirium predictor. Six of 14 studies found no relationship between sleep and delirium: two PSG studies ^{19, 34} , three using subjective measures ³⁴ , and one with an unspecified method ^{37, 40, 46} . One study found no difference in the rate of delirium between patients with typical and atypical sleep on PSG ³⁹ , while another by Boesen et al. also found no difference in atypical PSG results between patients who did or did not develop delirium ³⁴ . They compared PSG results with clinical behavioral observations and were only able to ascertain that the more pathological the patient and electroencephalogram findings, the less association with observed sleep ³⁴ . A study using the Richards-Campbell Sleep Questionnaire (RCSQ) found no significant correlation between perceived sleep quality and delirium, nor any significant relationship when asking how disruptive noise was to sleep ³⁷ . The study by Davoudi et al. ⁴⁶ used the Freedman Sleep Questionnaire and found no correlation between overall sleep quality and delirium, although they noted patients with delirium were more likely to have difficulty falling asleep and find night-time lighting disruptive ⁴⁶ . The last study did not detail their methodology, but found delirium was not significantly related to sleep deprivation ⁴⁰ .

Of the nine studies showing statistical correlation between sleep and delirium, three used electronic sleep monitoring ^{36, 42, 43} , five subjective survey measures ^{35, 41, 45, 47, 62} and one did not specify the methodology used ⁴⁴ . One study found atypical sleep on PSG was significantly tied to increased delirium, while another PSG study found delirium was associated with severe rapid eye movement (REM) reduction ^{36, 42} . A third study used a novel sleep monitoring device and found a relationship between a lack of REM sleep and delirium ⁴³ . Their results must be taken in the context of the device being commercially unavailable (Zeo wireless sleep monitor), and the authors not reporting statistical analyses. Among the remaining positive correlational studies, two had patients self-report sleep satisfaction and quality and both saw significantly poorer responses when comparing patients who developed delirium with those who did not ^{35, 45} . Two studies involved nursing staff observing clinical behaviors and found sleep disturbances were positively linked to higher likelihood of developing delirium ^{38, 41} . Two studies found an association between delirium incidence and sleep deprivation (methodology not specified) ⁴⁴ , and between sleeping disorders and delirium development ⁴⁷ .

In total, 16 studies evaluated the effects of a modified environment on delirium prevention or management ^{7, 14, 15, 18, 48– 59} ( Table 5– Table 7). Half were randomized control trials (RCT) ^{18, 49, 51– 54, 56, 57} , while half used different study designs including: before-after ^{7, 14, 48, 59} , retrospective cohort ^{15, 50} , and prospective cohort ^{55, 58} . Sample sizes varied from 11 to 748. Interventions focused on controlling environmental risk factors, including noise and light exposure, disrupted circadian rhythm, and sleep ( Figure 2). We categorized these interventions into four modification types: architectural design ^{18, 48} , environmental noise ^{14, 49} , environmental light ^{15, 50– 57} , and environmental modification bundles with noise and light components ^{7, 57– 59} . A summary of environmental interventions on delirium and reported statistical results are presented in Table 8. The interventional articles with results on delirium modifiable risk factors such as noise, light, and sleep were excluded if they did not assess delirium as an outcome. Table 9 represents list of these excluded studies.

Architectural design. Two studies ^{18, 47} explored a modified ICU design. One study altered the acoustics of the ICU ¹⁸ , whereas the other used a multi-aspect architectural design intervention ⁴⁸ . Results were mixed, but subtly suggest the benefit of architectural designs that consider acoustic features ( Table 5). Zaal et al. ⁴⁸ assessed patient outcomes in a multi-bed ICU room with less natural light and more noise exposure versus a private room with improved daylight and reduced noise by sound absorbers, glass sliding doors, optimized alarms, and remotely controlled monitors. There was no effect on delirium incidence or severity, but they found a reduction of delirious days in the study group by 0.4 (95% confidence interval (CI) 0.1–0.7, p = 0.005). Another quasi-randomized feasibility study ¹⁸ conducted noise reduction by refurbishing an ICU room. They installed a wall-to-wall drop ceiling, low frequency sound absorbers, and used a visually plain design. Implementing the noise reduction strategies was deemed feasible, requiring improvements in noise measurements and delirium assessments. Given the small sample size (n=31) and feasibility nature of the study, no further statistical analysis of outcomes was performed; Delirium developed in 33% (2/6) versus 25% (5/25) of study versus control patients. There was a slight reduction in noise reverberation and increase in speech clarity in the modified room, though sound levels remained higher than the WHO recommendations ¹⁷ .

Noise modification. In this review, there were two approaches to mitigate patient exposure to excessive sound. One was to reduce source noise by utilizing behavioral strategies and device/alarm optimization. The other was noise abatement by earplugs. No studies investigated the impacts of behavioral modification on delirium as an independent intervention, but this strategy was used as part of an environmental modification bundle in four studies ^{7, 14, 58, 59} . Earplugs were mostly a component of an environmental bundle ^{7, 14, 57, 58} , though one study evaluated the effect of earplugs as a single-component intervention ⁴⁹ . One article implemented a combination of behavioral strategies and earplugs to reduce excessive noise ¹⁴ . There were mixed findings across studies with noise modification component(s), but results suggest behavioral strategies and earplugs together might help delirium prevention, particularly as part of a multi-disciplinary program targeting environmental risk factors. However, the implementation of sustained behavioral changes and tolerability of earplugs remain challenges ⁵⁷ ( Table 6).

Van de Pol et al. ¹⁴ analyzed the impact of noise reduction on 421 non-delirious ICU patients in an interrupted time series before-after study. They used earplugs and behavioral strategies, including limited bedside conversations, lowered voices, grouped care activities, optimized alarm settings, and closed doors. Reported noise levels were still higher than the WHO limit post-intervention ¹⁷ , however there was a significant decrease in delirium incidence by 3.7% per time interval (p = 0.02), and reduction in sleep medication usage (p < 0.0001) in the study group. Perceived night-time noise was improved, but with no effect on sleep quality or use of delirium medication. Van Rompaey et al. show associations between environmental noise, sleep perception, and delirium ⁴⁹ . They conducted a randomized control trial on 136 non-delirious ICU patients and found use of earplugs (from 2200 to 0600) reduced risk of confusion or delirium by 53% (hazard ratio 0.47, 95% CI 0.27–0.82) and improved sleep perception.

Our full-text review and data extraction appraised articles studying single-component noise control strategies, such as behavioral programs ^{65– 69} , earplugs or noise cancelling headphones ^{71– 74} , and headphones equipped with an alarm filtering system ⁷⁰ ; however these were not included since they reviewed the impact of interventions on the level of noise or quality of sleep, but delirium was not reported as an outcome ( Table 9).

Light modification. Light interventions were implemented in an attempt to realign circadian rhythms by reducing night-time exposure and/or improving natural or artificial daylight exposure ( Table 7).

Reduction of nocturnal light exposure

The included articles in this review, studied eye masks ^{7, 57, 59} and overnight light dimming ^{7, 58, 59} as part of an environmental modification bundle to reduce night-time light exposure. However, the effects of less nocturnal light exposure on delirium, was not evaluated as a single intervention.

Improving natural daylight exposure

Three observational studies ^{31, 33, 87} and one before-after study ⁴⁸ investigated improved natural lighting via windows. They compared patient outcomes in rooms with a window or larger-sized windows versus windowless or smaller-sized windows, respectively. No observational studies suggested association between improved natural lighting and delirium ^{31, 33, 87} . Zaal et al. ⁴⁸ demonstrated reduction in delirium duration, comparing patients in private rooms with more natural light versus less bright multi-bed rooms; however, there were no differences in delirium incidence or severity between groups.

Improving artificial daylight exposure

Eight studies examined effect of improved daylight exposure via artificial lighting, of which three used an artificial circadian lighting system ^{15, 50, 51} , and five used bright light therapy (BLT) ^{52– 56} . No study implementing artificial dynamic or circadian lighting revealed significant effects on delirium. BLT studies had mixed results; three studies significantly improved delirium prevention or management, while the other two showed a non-significant tendency to reduce delirium rates.

A retrospective cohort study of 183 non-sedated ICU patients by Estrup et al. ¹⁵ used a circadian lighting system from 0700 to 2300 which varied in intensity and color temperature. During the morning, light intensity was greatest, up to 4000 lux (lx), and the amount of blue light strongest. As the day progressed, light intensity decreased and color temperature shifted towards warmer tones until no blue light was present. There was no improvement in delirium incidence, and no association between circadian lighting and delirium incidence (odds ratio (OR) 1.14; 95% CI 0.55, 2.37; p = 0.73). Pustjens et al. ⁵⁰ retrospectively studied a cohort of 748 non-sedated patients. They implemented a dynamic lighting system consisting of two ceiling-mounted light-emitting diode (LED) panels which delivered variable intensities of light (peak of 750 lx) with a color temperature between 2700 and 6500 Kelvin (K). There was no effect on delirium incidence. Another RCT by Simons et al. ⁵¹ measured the effects of a dynamic lighting application (DLA) in 734 ICU patients. DLA was administered through ceiling-mounted fluorescent lights which delivered a variety of bluish-white light from 0700 to 2230 with a maximum intensity of 1700 lx and a maximum temperature of 4300 K between 0900 and 1600, except between 1130 and 1330 when light intensity was 300 lx. This study was terminated early, but preliminary analysis demonstrated delirium incidence of 38% versus 33% in control versus study patients, with no significant improvement on delirium incidence or duration.

Four studies investigated the use of BLT as a single-component intervention to prevent ^{52, 53, 56} or treat ⁵⁴ delirium, while one study used BLT as an element of a multi-component bundle to manage delirium ⁵⁵ . BLT consisted of exposure to high intensity light (2000 to 10000 lx) for one to four hours daily. Three studies used a peak intensity of 5000 lx ^{52, 53, 56} . Taguchi et al. ⁵² conducted a randomization pilot study on 11 post-operative patients, utilizing a daily light intensity of 5000 lx from 0730 to 0930 for days 2 through 5 post-surgery. Delirium assessment scores decreased on day 3 of BLT (p = 0.014), but there was no significant effect on overall delirium incidence (16% versus 40% study versus control group, p = 0.42). In another RCT, Ono et al. ⁵³ applied BLT on 22 post-operative patients, for two hours from 0730 to 0930 for four days. Light intensity started at 2500 lx, increasing to 5000 lx, then decreasing to 2500 lx. There was a non-significant tendency towards lower rates of delirium in the study group (1 of 10 patients) versus control group (5 of 12 patients), while BLT significantly reduced the amount of activity during sleep on days 4 and 5. Potharajaroen et al. ⁵⁶ studied 62 post-operative patients by implementing BLT at 5000 lx from 0900 to 1100. Eleven of 31 control patients versus 2 of 31 patients in the intervention group developed delirium. There was a significant association between BLT and decreased delirium incidence (OR 0.12, 95% CI 0.03–0.54, p = 0.005). A study by Yang et al. ⁵⁴ on 36 delirious patients used a higher light intensity (10000 lx) over a shorter period (0700 to 0800). This study investigated the use of BLT as an adjunctive treatment of delirium with risperidone. They found a significant decrease in delirium severity in patients receiving BLT in addition to risperidone (DRS 23.9 ± 4.9 versus 20.6 ± 3.6 in control versus study group, p = 0.03). Chong et al. ⁵⁵ studied 228 delirious elderly patients admitted to a delirium management unit. They incorporated lower intensity BLT as part of their multi-component program, and exposed patients to 2000 to 3000 lx of light for four hours from 1800 to 2200 daily. They reported significant improvement in total sleep time and functional outcomes during treatment of delirious patients.

Intervention bundles (combination of light and noise modification)

Earplugs and eye mask

One reviewed study explored effects of earplugs and an eye mask on delirium ⁵⁷ , while two others used earplugs and an eye mask as part of their interventional bundle ^{7, 59} . All three noted decreased incidence of delirium, but observed different effects on sleep quality. Demoule et al. ⁵⁷ conducted an RCT on 43 non-sedated ICU patients to investigate the impact of sleeping with earplugs and an eye mask from 2200 to 0800 on patient outcomes. They found no improvement in delirium incidence, duration or architecture of sleep in the study group, regardless of patient compliance using the equipment. Although compliant study subjects experienced improved sleep with longer N3 (deeper sleep) duration and a lower number of prolonged awakenings, there was no significant change in delirium incidence. There were several articles in our initial screening reporting improved perceived noise or sleep quality with the use of earplugs and eye masks, however those were excluded since none reported results on delirium ^{78– 77} ( Table 9).

Quiet time, and sleep promotion bundles

Quiet time is a specific amount of time during which modifiable noise and light is actively reduced. Our review included three studies installing quiet time as the single interventional element ⁵⁸ or as a part of a sleep promotion bundle ^{7, 59} . Core elements of quiet time were behavioral strategies, minimized bedside activity by clustering care, reduced volume of devices/alarms, and dimmed lights ^{7, 58, 59} . The study that implemented daytime quiet time failed to show significant effects on delirium ⁵⁸ , while two sleep promotion studies decreased delirium incidence using nocturnal quiet time combined with components such as earplugs, eye masks, and pharmacological targets ^{7, 59} . Although the multi-component sleep promotion trials decreased delirium incidence, effectiveness of the separate components is unclear.

McAndrew et al. ⁵⁸ applied quiet time from 1400 to 1600 among 72 mechanically ventilated ICU patients. In the 24 hours after starting quiet time, there was no increase in delirium rate and 19% of delirious patients improved to a negative CAM-ICU status. However, there was no significant effect on delirium in their analysis. Quiet time did lead to moderately improved sleep quality and less frequently administered sedatives which helped remove patients from mechanical ventilation. A pre-post research by Patel et al. ⁷ studied a nocturnal multidisciplinary environmental sleep promotion program in 338 non-delirious, non-sedated ICU patients. Their program included nocturnal quiet time with earplugs, eye mask, patient orientation, early mobilization, and sedation targets. The study group showed significant reduction in delirium incidence (by 33% p < 0.001), and a decrease in delirium duration (3.4 ± 1.4 versus 1.2 ± 0.9 days, p = 0.021). Sleep quality and night-time light and noise levels were also improved in the study group, however reported noise levels were still higher than the WHO limits ¹⁷ . They additionally reported a significant association between sleep efficiency and a lower risk of developing delirium (OR 0.90, 95% CI 0.84–0.97). A larger pre-post study (n=300) by Kamdar et al. ⁵⁹ initiated a multi-faceted sleep promotion protocol consisting of three additive stages: 1) nightly quiet time and realignment of circadian rhythm, 2) sleeping with earplugs, eye masks, and soothing music, and 3) pharmacological targets to reduce sedatives. They reported decreased delirium incidence (OR = 0.46, 95% CI 0.23–0.89, p = 0.02) and perceived night-time noise in the study group, but no improvements in sleep quality.

ICUs are high-tech environments with round-the-clock activities that have a negative impact on patients’ experience and clinical outcomes due to excessive noise, light, and disturbed sleep and circadian rhythm ^{13, 48, 49} .

Noise. The WHO set recommendations for hospitals not to exceed an average of 30 dBA or a maximum of 35 dBA in treatment areas (maximum of 40 dBA at night) ¹⁷ . A 2016 study by Hu et al. found average sound levels of 62.8 dB, with a mean level of 59.6 dB between 0000–0700, when investigating sound in various ICUs ⁸⁸ . Consistently, five reviewed articles measuring ICU sound pressure with or without noise modification interventions reported levels exceeding the WHO recommendations ^{14, 18– 20} .

A 2009 WHO report set night-time noise guidelines and reported on relationships between night-time noise, sleep, and health. According to the report, excessive night-time noise (above 35 dB) disturbs sleep, provokes annoyance and agitation, reduces cognition, impairs communication and comprehension of surroundings, and contributes to psychiatric disorders. The combination of sleep disruption, decreased cognitive function, and lowered comprehension of surroundings associated with high noise levels may contribute to acute confusion and delirium ^{89, 90} . In our review, two of three observational studies investigating the association between high noise levels and ICU delirium found that high noise levels had no significant effect on delirium incidence ^{19, 20} . This result is surprising as it has been suspected that noise levels exceeding a normal threshold have detrimental effects on patient recovery, especially with regard to sleep and mental status. It is worth considering the difficulty in assessing the true effect of high noise levels in these two studies. First, there is no available baseline research to compare delirium incidence in high noise level ICUs versus those with statistically lower decibel values. It is possible the threshold for adverse effects is lower or higher than the most recently investigated decibel levels. In addition, Knauert et al. ¹⁹ mentioned a limitation for their study in the inadequate statistical power to detect differences in decibel level between patient comparisons. For the study by Johansson et al. ²⁰ , their results need to be taken in context of using a non-validated delirium diagnosis protocol.

Light. During the daytime, normal light intensity is around 10000 lx and recommended night-time light levels conducive to sleep are below 30 lx ⁶⁰ . Natural fluctuation of light levels throughout the day contributes to the natural sleep-wake cycle by triggering the release and suppression of melatonin. Alteration of the sleep-wake cycle and a lack of daylight schedule have been shown to be associated with psychiatric diseases ⁶⁰ . Daytime light levels in the ICU are below normal daylight levels and above the threshold for sleep disruption at night ⁶⁰ . In a study by Hu et al. light intensity was measured over 24 hours near windows, in the center of rooms, and at the eye level of mechanically ventilated patients. Average light intensity at these locations were 425 lx, 191 lx, and 388 lx respectively over 24 hours and 84 lx, 103 lx, and 87 lx between 2401 and 0759 ⁸⁸ . Minimal variation in daytime and night-time light levels disrupts the natural sleep-cycle and may contribute to patients becoming unable to distinguish day from night.

Abnormal natural light cycles are cited in recent literature as a potential modifiable risk factor for delirium management ⁶⁰ . Seven studies analyzing the impact of natural light on delirium incidence suggest this element of the ICU lacks a definitive causative relationship with development of the condition. Most of these studies enrolled critically ill patients whose condition gives them a higher likelihood of having consistently closed eyes compared to the general hospital population. It should be considered for future research that these patients’ retinas may not receive the same strength light stimulus as other populations, suggesting the need for ICU-specific lighting strategies. For the two seasonal studies, one found delirium was diagnosed significantly more in the winter than summer ³⁰ , while the other found exhaustive evidence ruling out a link between delirium and pre-hospital photoperiod exposure year-round ³² . These findings suggest there are factors aside from seasonal light exposure affecting delirium. Additionally, of the three studies with a positive correlation between exposure to natural daylight or season of admission, the two natural daylight studies had vague descriptions of their measurements of patient’s exposure to natural or artificial light ^{13, 47} . It is hard to assess whether the patient could have received benefits when the proximity of the stimulus to the patient is unclear.

As with excessive noise levels, further research into abnormal natural lighting cycles is necessary to delineate any threshold for adverse effects to patients’ well-being.

Sleep. Similar to our findings regarding effects of noise and light levels on delirium, reviewed articles on sleep showed mixed results for both forms of measure (electronic sleep monitoring and subjective reports). Recent literature states sleep is disturbed in ICU patients regardless of delirium ^{19, 42} , and this concern is supported by the fact that unmeasurable sleep was found in non-delirious patients in included PSG studies. It is hard to compare results of included wireless monitoring studies, since different methodologies were used for each study, with different devices, leads, and levels of adherence to American Academy of Sleep Medicine standards. Similarly, it is difficult to compare findings from objective sleep monitoring protocols and subjective survey methods, and these need separate consideration. A major concern in analyzing subject sleep quality in delirious patients is patients with an altered mental state and/or confusion may not answer consistently or truthfully, and measures must be taken to assess whether answers are a correct representation of their condition.

Noise modification. The negative impact of patient exposure to noise led to several studies focusing on noise pollution in the clinical environment. Mitigated exposure to noise levels might promote patient outcomes and staff satisfaction ^{58, 91} . Noise reduction or abatement strategies include architectural features, behavioral alterations, alarm optimization, earplugs, headphones, and noise cancelling devices. Whilst these strategies have been studied in relation with improved noise levels and sleep promotion ( Table 9), further research is required to make evidence-based recommendations for the effect of noise reduction on delirium prevention and treatment.

Implementing ICU designs with acoustic features such as sound absorbers, reversible drawers to open both inside and outside the room, or room designs with the ability to locate alarmed devices or transfer alarms away from the patient, might improve exposure to noise and benefit delirium management ^{48, 92, 64} . Zaal et al. demonstrated a lower delirium duration by modifying ICU design with acoustic considerations, however there was no change in delirium incidence rate ⁴⁸ . These strategies require major renovation or early construction planning, and further research is required to confirm cost-effectiveness and clinical benefits.

Staff and family conversations and care-activities are significant sources of ICU noise pollution ^{16, 65, 66} . Although behavioral modification might be ineffective as a single-component intervention ⁶⁷ , low-cost adjustments such as limited bedside conversation, lowered voices, clustered care-activities, minimized TV and overhead use and volume, use of vibrating pagers, and visual noise-warning devices may be necessary to achieve better results in sound reduction ^{7, 14, 65, 66, 68, 69} , sleep improvement ⁶⁶ , and decreased delirium ¹⁴ . To be successful, continuous awareness, education of staff on the impact of excessive noise exposure, and routine monitoring of implemented strategies is crucial ⁷ . Technologies that help staff and visitors recognize excessive noise might complement implementation of behavioral strategies. Visual noise-warning devices display colored warnings at higher levels of noise and can be an effective, sustained noise reduction strategy ^{68, 69} . Use of noise-warning systems has a greater impact on the reduction of ambient noise compared with peak noise levels ^{68, 69} . This is likely a result of change in staff behavior after visual warning while having no effect on medical equipment or alarms.

Alarms are a significant source of ICU noise pollution ^{16, 65} , and a large portion are considered false positives ⁹³ . Studies show modifying ICU alarms by lowering volume, optimizing device settings, and filtering false alarms may reduce disturbing alarm noise ^{94– 80} . Schlesinger and colleagues equipped wearable earbuds with a frequency-selective silencing device, which could successfully filter ICU alarms while allowing patients to hear and communicate effectively without experiencing the negative consequences of audible alarms ⁷⁰ . Optimization of alarms was used as an element of a noise reduction bundle and sleep promotion studies of this scoping review ^{7, 14, 59} .

Abating environmental noise by earplugs or headphones appears feasible and effective to reduce noise and improve sleep in the ICU ^{49, 57, 71– 74} . Here, one study failed to prove benefits of using earplugs and eye masks during sleep on delirium ⁵⁷ , while another earplug trial decreased risk of confusion, and delayed initiation of cognitive disturbances with no significant effect on incidence of delirium ⁴⁹ . Given the potential effectiveness and low costs, this method is frequently used in multi-component interventions ^{7, 59} ; however, non-compliancy is an issue in earplugs studies ⁵⁷ . A recent meta-analysis ⁹¹ reported a 13.1% (95% CI, 7.8–25.4) rate of non-compliancy due to intolerance, anxiety, or accidental removal of earplugs. Headphones with active noise cancellation technology might improve patient outcomes by mitigating exposure to noise. Gallacher et al. modeled an experiment by embedding sound meters in the auditory meatus of polystyrene model heads located near patients’ beds in a cardiac ICU ⁷⁴ . They demonstrated a significant reduction in overall noise exposure and exposure to high intensity noises using noise cancelling headphones.

Despite inconsistent results of the reviewed studies on efficacy of noise modifications on delirium, this review suggests considering physical design features and multi-component noise reduction programs may benefit delirium prevention or management. This is consistent with current recommendations suggesting multi-component interventions to achieve adequate noise reduction ⁹¹ ; Van de Pol et al. ¹⁴ reduced delirium incidence by implementing a noise reduction program consisting of behavioral strategies, device optimization, and earplugs. However, there is a need for high-quality randomized control trials with larger sample sizes to evaluate efficacy, sustainability, and long-term effects of noise modification interventions with a focus on delirium.

Light modification. Optimized circadian rhythm needs bright days and dark nights. Various light modification strategies have been proposed to follow circadian rhythms. These are categorized as such: decreasing night-time light exposure, and increasing daylight.

Round-the-clock ICU activities make nigh-time light reduction challenging to maintain a level of light sufficient for providing care, but not disturbing sleep. Dimming lights as part of quiet time strategies is effective to mitigate intensity of light during quiet time hours, however, this may cause variation in perceived light and consequently cause sleep disturbance ⁸⁰ . Possible solutions are clustering care-activities to reduce bedside interruptions ⁷ and use of portable lighting pods with less blue wavelength during the night ⁷⁹ . Whilst the trial of sleep masks and earplugs by Demoule at al. ⁵⁷ failed to show benefit to delirium, eye masks are effective in promoting sleep by light abatement ^{7, 78, 76, 77} . However, poor compliance in the use of eye masks due to accidental removal, or anxiety/claustrophobia, and the risk of sensory deprivation in mechanically ventilated patients, remains challenging ⁵⁷ .

Environmental modification to increase daylight exposure is possible through the architectural considerations of promoting natural lighting or utilizing artificial illumination. Research into whether windows allow enough light to promote sleep-wake cycles and prevent delirium, and whether seasonal light levels contribute to delirium, has been conducted with inconclusive findings ^{31, 33, 87} . From our results, the greatest interventional effect on delirium was from bright light therapy.

Our review included five studies on BLT, three reporting a significant effect on delirium incidence or severity ^{52, 54, 56} with sleep promoted in four studies ^{53– 56} . BLT has the greatest effect between 2500 and 10000 lx for 30 to 60 minutes, with a shorter duration for greater intensities of light, when administered either at twilight or dawn to obtain a circadian effect ⁶¹ . The BLT in this review applied 2000 -10000 lx of illuminance for between one and four hours. The use of 2000 lx was effective in improving sleep quantity and functional status during management of delirium as part of a bundle. The use of 5000 lx was associated with decreased delirium incidence in two of three studies and the use of 10000 lx, as an adjunctive treatment with risperidone, was associated with a decrease in delirium severity ⁵⁴ . While BLT may help regulate sleep-wake cycles and prevent/treat delirium, research into melatonin secretion and circadian rhythms suggests periods of darkness play as large a role as daytime light levels in promoting sleep and preventing delirium ^{60, 95} . The importance of light and darkness prompts a need for research into effects of dynamic lighting systems. This review included three studies focused on dynamic lighting among sedated and non-sedated patients, using lighting systems which produced cooler blue light in the mornings and shifted towards warmer tones as the day progressed. The lighting systems produced different levels of intensity throughout the day, reaching a peak of between 750 and 4000 lx and a minimum level of 0 lx. None of these studies showed significant effects on delirium ^{15, 50, 51} ; however, they used peak light levels below normal daytime levels.

Maintaining a circadian rhythm, by nocturnal darkness and BLT, as a low-cost, low-risk, easy-to-apply intervention can help improve patient outcomes. Research is required to investigate the use of dynamic lighting with higher peak light intensities or the combination of dynamic lighting and BLT. Additionally, there is a need for defining effective characteristics of light modification strategies for sedated and non-sedated patients. Sedated patients may have disrupted circadian rhythm of melatonin ⁹⁶ , and application of light therapies might have limited retina stimuli when eyes are closed. Studies comparing efficacy of light modifications on prevention or treatment of delirium among these two groups of patients, with application of different intensity levels of light in closed-eyes patients, might be of benefit.

Intervention bundles (light and noise modification). There is a growing interest in using quiet time interventions to promote sleep. Quiet time protocols have successfully reduced sound pressure, improved sleep quality, and reduced the use of sedatives ^{80, 81– 83} , but effects of quiet time on delirium development needs further research. McAndrew et al. implemented a daily quiet time protocol in ICU patients and reported inconclusive results on delirium scores and moderate improvement in sleep perception ⁵⁸ . Two neurocritical ICU studies have implemented a two hour quiet time during day and night ^{81, 82} . A significant improvement in subjective sleep and increased staff satisfaction was achieved ^{81, 82} . They reported decreased light by 75–85% and noise by 15%, with results being more significant during day-shift quiet time; this might be due to overall lower levels of nocturnal light and noise ⁸² .

Sleep promotion protocols utilize noise and light control strategies with other components, such as patient orientation, early mobilization, medication optimization, and sedation targets to improve sleep in quality and quantity. Here we included two sleep promotion studies reporting results on delirium, however future research is needed to evaluate which component of sleep promotions are effective in reducing delirium. Patel et al. ⁷ significantly improved sleep quality and reduced delirium incidence by implementing a non-pharmacological multidisciplinary sleep program. They raised protocol compliance to > 90% by ongoing education, signage and posters, monitoring, and spot-checking program quality by experienced nurse champions. Interestingly, a large sleep promotion study by Kamdar et al., decreased delirium incidence while there was no effect on sleep ⁵⁹ . It is not clear if improvements in delirium are attributable to sleep, emphasizing the need for future studies focused on single interventions or single components of multifaceted interventions with regard to delirium results.

The main strength of this review is synthesizing results of both observational association studies and interventional studies. This approach details a broader picture of the current state of this research field and bridges the gap between establishing correlational relationships and continuation of experimental trials. A major limitation of this review is the narrow search method. By searching one database (Pubmed) and the included articles’ reference lists, there is likely additional literature available to expand our findings, however the authors did a hand search within related journals, Embase, and Google Scholar databases to include existing interventional research articles. Another limitation was that the generated data from reviewed studies did not have full details, and quality of evidence was not evaluated among studies; however, this review was intended to be a literature mapping with limited description of relevant publications.

All data underlying the results are part of the article and no additional source data are required.

OSF: PRISMA-ScR checklist for ‘The Impact of Environmental Risk Factors on Delirium and Benefits of Noise and Light Modifications: A Scoping Review’. https://doi.org/10.17605/OSF.IO/NHWKA ²⁹

Data are available under the terms of the ‘Creative Commons Zero "No rights reserved" data waiver’ (CC0 1.0 Public domain dedication).