Yoon et al. [6] investigated whether alertness at night, executive performance, and morning sleep could be improved by using bright light exposure at night and morning light attenuation. Twelve nurses participated, and the investigators studied three conditions: 1) standard illumination during NSW; 2) bright-light illumination during NSW; and 3) bright-light illumination during NSW and the use of sunglasses the following day. In the first condition, standard hospital illumination was administered during NSW. In the second condition, the nurses were exposed to bright light of 4,000–6,000 lx for 4 h (1:00–5:00) during their NSW, followed by morning sunlight exposure. In the third condition, nurses received the same bright light treatment during the night shift but sunlight was attenuated the following morning by using sunglasses. Each treatment was performed for four days in a crossed design. Nighttime alertness was measured by using the Analog Visual Scale (AVS); a computer-based test was used to evaluate executive function, and morning sleep was measured by using actigraphy. Bright light exposure and wearing sunglasses led to significant improvements in the studied variables. Bright light showed less improvement than sunglasses but better results than standard illumination.

Tanaka et al. [7] investigated whether brief bright light exposure on the morning of a working day could improve the health, performance, and safety of nurses with fast rotatory shifts. Sixty-one nurses participated in this study. The subjects were instructed to expose themselves to bright light for 10 min on the morning of the shift day. Thirty-one participants received bright light in the first month, and the other 30 in the second month. There was a clinical improvement in the light-exposed condition, as reflected in self-reported daytime sleepiness (Karolinska Sleepiness Scale), night sleep quality during shift days (AVS), and fatigue assessed using the Checklist Individual Strength Questionnaire. The estimated differences for each scale (mean values and 95% confidence intervals [CIs]) were -0.55 (-0.91 to -0.20), 0.37 (0.04 to 0.70), and -2.13 (-3.78 to 0.48), respectively. The mean reaction time (RT) during the Psychomotor Vigilance Test (PVT) improved upon exposure to bright light. There were no differences in afternoon sleepiness, depression, number of pauses during the PVT, frequency of perceived adverse events, or near misses.

Huang et al. [8] investigated whether bright light exposure during the first half of the evening or night shift, combined with light attenuation in the morning, was effective in improving the sleep complaints of female nurses with insomnia during fast rotatory shifts. The investigators used the Insomnia Severity Index (ISI) and the Anxiety and Depression Scale (ADS). Nurses with an ISI score >14 participated. The participants under intervention (n=46) were exposed to 7,000– 10,000 lx light for 30 min or more for 10 shifts and wore sunglasses the day after the shift. Nurses in the control group (n=46) were not exposed to bright light but wore sunglasses after shifts. Participants avoided sunlight exposure the following day or wore sunglasses. After the intervention, significant improvements in ISI and ADS scores were observed.

Rahaman et al. [9] studied the effects of light filtration (<480 nm) during NSW on sleep and work performance in five female and four male nurses with a mean age of 31.3±4.6 years. The participants received treatment with random or standard indoor illumination. Nighttime sleep after two night shifts and daytime sleep between the two night shifts were measured by using polysomnography (PSG). In addition, melatonin levels were quantified in saliva, and alertness was determined every 2 h during the first shift and on the middle shift of the three night shifts. Sleep and performance at baseline and under the intervention conditions were compared with daytime performance on the seventh shift day, and nighttime sleep following the seventh shift day. In the basal PSG, total sleep time (TST) and sleep efficiency (SE) were decreased, whereas wake after sleep onset (WASO) was increased. After the intervention, TST increased by 40 min compared with baseline, WASO decreased, and SE increased. Melatonin levels were higher in the first and middle shift night of the intervention. The subjective sleepiness increased throughout the night under both conditions. RT and vigilance test scores were similar to daytime performance after treatment but were impaired during the first night shift. At the middle of the night shift series, the difference in performance was not significant between the day shift and the two night shifts.

Griepentrog et al. [10] investigated whether exposure to bright light during NSW reduces sleepiness and increases psychomotor performance in intensive care unit nurses. The participants were exposed to bright (1,500–2,000 lx) white light for 10 h, and their results were compared with those exposed to standard illumination. The nurses completed the Stanford Sleepiness Scale (SSS) and the PVT at the onset and end of the study. Forty-three couples were compared. Bright light exposure reduced somnolence on the SSS (mean 2.6±0.2 vs. 3.0±0.2); however, they had a higher frequency of errors (2.3± 0.2 vs. 1.7±0.2; p=0.03).

Olson et al. [11] studied the effect of a light intervention on 33 nurses in fast-rotating shifts during two separate periods, consisting of 2–4 consecutive night shifts and a two-day period before and after the shift. During the intervention, the nurses followed a fatigue reduction plan comprising 40 min of exposure to bright light from a portable light box before the night shift, avoiding light by using sunglasses after the night shift, and following a recommended sleep and nap schedule. After the intervention, the nurses reported less fatigue, fewer errors at work, better and more prolonged sleep, and a more positive mood. Participants with an evening chronotype reported more benefits.

Kjørstad et al. [12] compared the effects of blue-depleted light exposure (BDLE) and standard illumination in the workplace during shifts on sleep patterns between shifts. Twenty-five nurses participated in this study. The effects of the intervention were evaluated using actigraphy, self-reports of sleep (e.g., TST, SE), and shift functioning (e.g., mood, stress level, and caffeine use). The participants completed several scales to assess their physiological and mental health conditions and the side effects of each light environment. The authors observed a 17% within-subject reduction in sleepiness during evening shifts with BDLE compared with standard illumination (p=0.03; Cohen’s d=0.37). No differences in other sleep measures (sleep diary or actigraphy), self-reported stress, or mood levels were observed between conditions. In addition, nurses working with the BDLE perceived the lighting as warmer (p=0.009) and more relaxing (p=0.02) than standard illumination. However, there was little evidence of a positive effect of the BDLE on nurses’ sleep and work functioning.

Hoshi et al. [13] investigated whether attenuated room lighting produces less fatigue and sleepiness while working and better performance than bright white light. Light intensity under the two conditions was attenuated (110 lx) or bright (410 lx) in room lighting, over four months of exposure. Twenty nurses participated in the study. All the participants first worked with attenuated lighting followed by bright lighting. The participants completed a self-administered questionnaire at the beginning and end of the study. Fatigue and sleepiness were higher under attenuated lighting conditions than under bright lighting conditions (p<0.05). There were no differences in sleep quality, incidents, or accidents between the attenuated and bright illumination conditions.

Cyr et al. [14] studied whether light exposure before the evening of a night shift could improve fatigue, performance, mood, and sleep. Fifty-seven nurses in fast-rotating shifts participated in this study. They were randomly assigned to the experimental or control groups. The intervention consisted of evening and night light exposure and morning light avoidance. Under controlled light conditions, there was an improvement in alertness and a decrease in sleep alterations under a modified diet. Every morning and evening, the nurses completed measures of fatigue, work-related errors, sleepiness, mood, sleep duration, and sleep quality for 30 days. In the experimental group, errors were reduced by 67% compared with 5% in the control group. The light-exposed group reported less fatigue on workdays than controls (d=0.25 [0.10–0.38]). The experimental group also showed improvements in mood. Both groups showed improvements in fatigue (d=0.29 [0.20– 0.36]) and sleepiness (d=0.21 [0.12–0.28]), as well as a small decrease in sleep duration.

Liao et al. [15] studied the effects of low-energy light exposure on sleep, psychological symptoms, and heart rate variability (HRV) among nurses. The study included nurses with self-reported insomnia and NSW during the previous six months. The participants were randomized into the experimental (n=32) and control groups (n=32). A portable Meridian Aura Cap providing low-level red and near-infrared light (660 and 850 nm, respectively) for 30 min, three times a week, for four weeks was used. The control group did not undergo any intervention. The ADS and ISI scales were used. HRV was measured by using an ANSWatch. After four weeks of intervention, the experimental group reported lower scores for insomnia (4.3 vs. 12.6, respectively; p<0.001), depression (2.5 vs. 7.9; p< 0.001), anxiety (3.1 vs. 9.2; p<0.001), and stress (5.6 vs. 12.0; p< 0.001). No differences in HRV were observed.

This review shows that light-based interventions are plausible therapeutic techniques for addressing negative symptoms in nurses following NSW. These interventions could help alleviate sleep alterations resulting from sleep deprivation. Exposure to bright light during the night shift stimulates corticosubcortical networks that promote vigilance, and when it is followed by light amelioration or suppression, it activates the networks, promoting sleep and regulating periods of activity and rest [6,8,11]. In other studies, bright light exposure on the day of the NSW was used [7,10,14], and BDLE during the night shift was utilized with good results [9,12,15]. Two studies used a randomized sample selection [10,14], and the only complication reported was an increase in errors in work performance in a single study [10].

The light perceived by the retinal layer of the eye is transmitted through visual pathways. The accessory optic tract (retinohypothalamic system) connects the retina to the suprachiasmatic nuclei, with crossed and uncrossed fibers that project into the ventrolateral and posterior parts of this nucleus. The neurons of the nuclei have secretory and electrochemical properties that command the biological clock of circadian rhythms, which are adjusted every day by the sunlight coming into the eye [16]. Termoregulation, which is essential for sleep onset and continuity, is regulated by the preoptic area of the hypothalamus, which is influenced by the suprachiasmatic nuclei; some authors have found that the increase in light intensity stimulation alters heat loss at times when the circadian cycle promotes sleep, which desynchronizes the circadian cycle [17]. This dysregulation helps to explain why insomnia symptoms occur during the day after NSW. Simultaneously, filtering out blue light resulted in differential changes in melatonin and cortisol levels. Blue light exposure decreased nighttime melatonin levels, whereas blue and red light exposure affected cortisol levels [18]. This highlights the importance of reducing and filtering blue light during night shifts.

Alternating sunlight and nighttime darkness are the most important external mechanisms regulating the circadian clock and homeostatic equilibrium, and altering these cues can result in many pathophysiological disorders. However, NSW of healthcare workers is necessary to ensure the continuity of nursing for patients in hospitals and emergency services. Nevertheless, NSW alters the circadian rhythms and sleep quality of nurses, physicians, and technicians in hospitals and constitutes a high-risk variable for physical and mental alterations in nurses who work night shifts [19].

Yook et al. [20] evaluated the daytime electroencephalogram (EEG) patterns of nurses working night shifts and the changes induced by ambient light exposure (30 lx) vs. dim light (<5 lx). The study had 31 participants who worked night shifts and 24 controls who never worked in night shifts. The PSG and EEG variables were studied after nighttime sleep and daytime sleep with dim light and between daytime sleep with dim and 30-lx light conditions. They observed that the daytime sleep group showed lower delta power during non-rapid eye movement (NREM) sleep than the nighttime sleep group. During daytime sleep, the light exposure group was observed to have a lower sigma in N2 under light exposure, compared with no light exposure. During daytime sleep, lower slowwave power during NREM sleep during the last cycle was observed under light exposure compared with no light exposure. Data have shown that NSW and subsequent circadian misalignment strongly affect sleep quality in NREM sleep, and light exposure during daytime sleep could reduce N2 sleep spindle activity and N3 slow waves in the last sleep cycle, whereas PSG/EEG parameters during daytime sleep are better with dim light.

NSW results in a mismatch of biological clocks that can lead to physical (cardiovascular, gastrointestinal, metabolic, neoplastic, neurological, immunological, and others) [21], mental (stress, burnout, anxiety, depression, and others) [22], and cognitive alterations [23]. For example, in police officers, extended day and night shifts result in sleepiness, fatigue, circadian misalignment, and sleep disorders as the major causes of human errors, incidents, and accidents [24]. Thus, therapeutic interventions are needed to recalibrate the circadian rhythms and avoid complications.

However, there are some limitations that should be taken into account. There are few studies carried out on the subject, and the lack of utilization of the Cumulative Index to Nursing and Allied Health Literature (CINAHL) database could be another constraint.

Conversely, difficulties in blinding participants of the investigations or the small sample sizes of the studies found for this review might have influenced the integrated synthesis. This should be considered when decisions are made based on these results. Further investigation is required to overcome these limitations.

It is important for nurses to prevent complications associated with NSW. It is important to consider nonpharmacological interventions using light-based therapeutics to avoid the potential side effects of drug therapy on nervous system functions.

Two main actionable light-based interventions in the workplace seem feasible to reduce symptoms on the day after NSW in nurses. The first is the presentation of high-intensity illumination in work settings during NSW, followed by light attenuation on the following day with the use of personal sunglasses. This action drives the biological clock toward better sleep regulation with improved sleep architecture, quality, and restoration following fatigue [25]. However, it would require that lights in hospitals be replaced with newer energy-efficient lighting. The second would be to change lamps and illumination devices to those that filter out the blue color range, which would result in an improvement of the sleep alterations in nurses with NSW [26].

In addition to the light-based measures reviewed here, other lifestyle interventions have shown promise. For example, scheduling blocks of exercise into NSW followed by rest the day after [27], changing to a diet that promotes alertness in the evening prior to NSW, followed by foods that promote sleep for breakfast the following morning [28], and avoiding stimulants such as caffeine. These and other complementary measures could help nurses suppress the negative symptoms of NSW without requiring pharmacological intervention.

NSW symptoms, such as daytime somnolence and insomnia on nonworking nights, as well as physical and mental complications, could be experienced by night workers in a variable way depending on individual chronotypes and personality features [29]. More investigations must be performed to identify these characteristics to develop individually tailored treatments (light exposure, exercise, diet, etc.).