In developed countries, at least 15%-20% of the working population is engaged in shift work (Hossain et al., 2004). Disturbances in circadian rhythms may cause various physiological and psychological problems in shift workers, especially where night shift is involved. For example, shift work can affect body cortisol secretions, which, under normal secretion conditions, are high during the day and low at night. Cortisol is an essential hormone produced by the adrenal glands that affects many bodily functions, including metabolism and regulation of the immune system. Imbalances of cortisol are associated with fatigue, depression, obesity, and immune dysfunction (Kudielka, Buchtal, Uhde, & Wust, 2007). Compared with that of permanent day workers, cortisol secretion in night shift workers is lower in the morning and higher at night (Kudielka et al., 2007; Lac & Chamoux, 2004; Perkins, 2001). Night shift workers mostly sleep during the day. However, high cortisol secretion and high body temperature during the day can affect the quality of sleep and decrease the number of hours slept. Indeed, approximately 60% of shift workers complain about insufficient sleep and insomnia (Edell-Gustafsson, Kritz, & Bogren, 2002; Lamond et al., 2003; Muecke, 2005; Natale, Martoni, & Cicogna, 2003; Purnell, Feyer, & Herbison, 2002). Night shift workers generally feel more fatigued than daytime workers because the amount of sleep they get is usually insufficient, and their circadian rhythm is in a state of lethargy.
In addition, nonpermanent shift workers (i.e., those who work rotating shifts) experience chronic fatigue more often than permanent shift workers (Brooks & Swailes, 2002; Seki & Yamazaki, 2006; Winwood, Winefield, & Lushington, 2006). Fatigue and long-term poor sleep quality have been found to affect work performance, and although subjective cognitive ability is not considerably affected, objective cognitive performance for shift workers decreases significantly. Reaction time and the rate of critical incidents also increase during night work (Dingley, 1996; Lee, Chan, & Kwok, 2003; Smith, Folkard, & Poole, 1994).
Nursing staff are the largest group of professionals in the healthcare team that provide 24-hour patient care across a 7-day work week. There is an increased awareness that circadian rhythms in physiological and psychological processes are at a low ebb during the night. Work at the low point of circadian cycle increases the risk of night nurses being less alert than ideal. This may impair the efficiency with which they carry out their duties and endanger the lives of patients. The objective of this literature review was to investigate the influence of shift work on nursing staff cortisol profiles, sleep quality, fatigue, and attention levels. We propose that the results can be used as a reference point for nursing administrators to construct work schedules more suitable for maintaining shift worker health and safety.
The objective of this review was to examine the broad evidence regarding cortisol profiles, sleep quality; fatigue, and attention levels as they relate to shift work in general and to nursing shift work in particular.
The review included articles published from 1996 to 2008. The following electronic databases were searched: SCOPUS, OVID, Blackwell Science, EBSCO Host, PsycINFO, Cochrane Controlled Trials Register, CEPS, and thesis system. Key words used included shift worker or shift work or night duty or rosters, circadian rhythm disturbance or circadian rhythm, cortisol profile or cortisol, sleep deprivation or sleep quality, attention concentration or concentration or attention, and fatigue. The combined terms followed both MeSH terms and text words. Researchers also performed an extensive manual search and cross-referencing from reviews and original articles.
Inclusion criteria for the study were as follows: (a) full-time shift workers (including day, evening, night shift, and irregular rotation shifts), (b) shift work focus on cortisol profile change or sleep quality or fatigue or attention concentration, (c) English language, and (d) quantitative research articles.
Exclusion criteria included studies that identified a metabolic or neurological disease and a study focus on the exploration of hormones or hypnotic medication effect.
Quality assessment of randomized controlled trials and non-randomized controlled trials was performed independently by two reviewers. The methodological quality of eligible randomized controlled trials was assessed using the 6-point Quality Scale Assessment (QSA) tool developed by the Cochrane Collaboration. The methodological quality of non-randomized controlled trials was evaluated using a modified version of the Jadad Scale (Fernandez & Griffiths, 2004). All studies were ranked according to National Health and Medical Research Council levels of evidence (Table 1; National Health and Medical Research Council, 1999). Synthesis of data were presented in text and tabular format. Meta-analysis was not possible because of the heterogenic nature of reviewed studies.
A total of 28 key journal articles were included in the review. All articles selected for review were entered into a bibliographic software package (Endnote Version 11). A data extraction sheet was used to extract data from eligible studies (Table 2-5).
A total of 28 relevant journal articles were included in the review. Previous research on the effects of shift work on cortisol profiles, sleep quality, fatigue, and attention used data assessed at evidence Levels II to IV. One article involved a randomized controlled trial and 27 addressed non-randomized controlled trials. Study subjects included nursing staffs, physicians, healthy adults, factory workers, ambulance and train drivers, firefighters, locomotive engineers, policemen, and college students.
Effect of Shift Work on Cortisol
Cortisol secretion is controlled by the hypothalamic-pituitary-adrenal axis. Factors influencing cortisol secretion include diurnal rhythm, consciousness, and the sleep-wake cycle together with neural pressure signals. Normal cortisol secretion follows a negative slope. Cortisol secretion is lowest during Stage 1 sleep and increases gradually during Stage 2 sleep (Kudielka et al., 2007). The highest concentration of cortisol is reached the moment we wake up in the morning (Hennig, Kieferdorf, Moritz, Huwe, & Netter, 1998). Normal serum cortisol concentrations (approximately 250-850 nmol/L between 8:00 and 10:00 a.m.) maintain daytime consciousness then decrease continuously throughout the day (Putignano et al., 2001). The cortisol concentration at night is about half the daytime concentration, with a normal range of 110-390 nmol/L (Putignano et al., 2001). The range of cortisol concentration fluctuation is stable around the clock. However, increased cortisol secretion can be observed when an individual is under pressure or stress (Kudielka et al., 2007; Putignano et al., 2001).
Table 2 presents a summary of previous research on the effect of shift work on cortisol levels. According to the QSA tool developed by the Cochrane Collaboration, the levels of evidence in studies collected were Level III-2 to Level IV. Study subjects included nursing staff, healthy men, factory workers, and ambulance drivers. In these studies, saliva, blood, and urine samples were used to measure cortisol.
The time at which peak cortisol level occurs (cortisol acrophase) was observed at 6:00 a.m. in day shift workers, 7:00 a.m. in evening shift workers, and 11:00 a.m. in night shift workers. From studies conducted investigating cortisol levels, a typical cortisol-awakening rise (CAR) was observed in night shift workers. However, the average salivary cortisol acrophase of the night shift worker in the morning was 11 nmol/L. This level was lower than that of day shift workers (16 nmol/L). The CAR profile of night shift workers decreased at night. Abnormal CAR was observed in subjects who changed from day shift to rotating shifts. When night shift workers sleep during the day, their sleep cycles are reduced, and sleep quality is poor because of high cortisol concentration and low melatonin levels (Holmback et al., 2003; Kudielka et al., 2007; Lac & Chamoux, 2004; Mitani, Fujita, & Shirakawa, 2006). Circadian rhythm studies of shift workers also report that circadian rhythm is completely adjusted in shift workers after about 7 days, especially among shift workers transferring from day shift to night shift. Similar to circadian rhythm, the increase of cortisol concentration in day shift workers starts at 6 a.m. From Day 1 to Day 4 of the night shift, cortisol levels decreased between 9 p.m. and 12:00 a.m. and increased between 12:00 and 6:00 a.m. On Day 5 of the night shift, the cortisol levels start to increase at 9 p.m. In consecutive night shifts, cortisol concentration is higher in the afternoon than that in the morning, and reverse rhythm and night shift adjustments have been observed (Hennig et al., 1998; Lac & Chamoux, 2004).
Effect of Shift Work on Sleep Quality
There are five stages in each cycle, with four to five cycles in a typical night's sleep. Sleep Stages 1 to 4 are non-rapid eye movement (NREM) stages, whereas Stage 5 is a rapid eye movement (REM) stage. Sleep starts from being awake to a light sleep stage (Stages 1 and 2), then progresses to the deep sleep stage (Stages 3 and 4), and finally the REM stage (Stage 5). Each sleep cycle takes an average 90 to 100 minutes to complete. NREM Stages 1 and 2 were found to be shorter among long-term night shift nursing staff. In a normal physiological situation, melatonin induces sleepiness in a dark environment, and cortisol secretions maintain daytime consciousness during the morning. In general, peak sleepiness occurs between 3:00 and 6:00 a.m. However, because night shift workers sleep in the daytime, inconsistent with normal circadian rhythm, their sleep is easily interrupted by environmental factors such as light, noise, temperature, and telephone noise (Frey et al., 2002; Kudielka et al., 2007).
Because of these and other sleep disturbing factors, night shift workers are quite often sleep deprived. A study on sleep deprivation reported that when subjects were awakened during the REM stage of sleep, 60% to 90% said they were dreaming at the moment of awakening. Selective deprivation of the REM sleep stage was found to result in emotional instability and restlessness in such research subjects (Kudielka et al., 2007).
Table 3 presents data from previous research on the effect of shift work on sleep quality. According to the QSA, levels of evidence in these studies ranged from Level III-2 to Level IV. Study subjects included healthy women from the community, nursing staff, train drivers, railway traffic control staff, air traffic control staff, policemen, locomotive engineers, and pharmaceutical factory workers. Tools applied to measure quality of sleep included polysomnography, actigraphy, a subjective evaluation questionnaire, a sleep diary, the Sleep Hygiene Practice Index, the Epworth Sleepiness Scale and Pittsburgh Sleep Quality Index, and the Karolinska Sleepiness Scale.
Sleep deprivation can accumulate, and large sleep deficits may result in chronic fatigue (Edell-Gustafsson et al., 2002; Lamond et al., 2003; Natale et al., 2003; Purnell et al., 2002; Rotenberg, Moreno, Benedito-Silva, & Menna-Barreto, 1998). Reduced length of daytime sleep in night shift workers affects sleep quality and causes increased fatigue. Thus, increasing daytime sleep duration on the first day of night shift work can decrease fatigue and subsequently improve shift workers' overall sleep quality. Encouraging shift workers to sleep longer on their first day of sleep after starting night duty is therefore recommended. Compared with regular shift workers, irregular shift workers exhibited abnormal sleep patterns and shorter sleep times (Kudielka et al., 2007; Neylan et al., 2002; Roach, Reid, Ferguson, & Dawson, 2006; Rotenberg et al., 1998). Compared with day shift, the severity of sleepiness in the night shift is 6 to 14 times higher. It was 0.7 times as high in evening shift workers and 2.3 times higher in the early morning shift workers (Harma et al., 2002). Work duration increases of 1 hour resulted in an increase in degree of sleepiness of 15%. Sleep duration increases of 1 hour resulted in a decrease in degree of sleepiness of 15% (Harma et al., 2002). Evidence suggests that adjusting working hours and rest times for shift workers can decrease degree of sleepiness. In addition, a short sleep before starting duty can decrease the nighttime electroencephalogram delta wave and increase alertness (Frey et al., 2002). In the studies reviewed, sleep quality was significantly related to change in sleep patterns. Shift workers were more likely to drink excessively, smoke, drink coffee, and take hypnotics and sedatives than non-shift workers. A total of 60% of nursing staff used sleep aids, whereas 62.7% reported taking prescription medications and 26.9% drank alcohol to sleep (Dorrian et al., 2006). Studies indicated caffeine and alcohol intake as significant predictors of sleep quality. Subjects who drank more than two cups of coffee or more than one glass of alcohol per day and subjects who were ill felt less vigorous after sleep and had poorer sleep quality. Sleep quality and physical condition after sleeping were closely related to incidences of work-related error (Cheek, Shaver, & Lentz, 2004; Dorrian et al., 2006; Neylan et al., 2002).
Shift Work Fatigue
Irregular sleep patterns and interruption of night sleep can cause sleep deprivation, fatigue and physical collapse and affect physiological functions. When fatigue increases, shift worker reaction times increase and attention and judgment decrease (Samaha, Lal, Samaha, & Wyndham, 2007; Takeyama et al., 2005; Winwood et al., 2006).
Table 4 summarizes previous research on fatigue produced by shift work. According to the QSA, the levels of evidence were at III-2 to IV. Research subjects in these studies included young healthy individuals, nursing staff, and firefighters. The Fatigue Severity Scale, the Occupational Fatigue Exhaustion Recovery Scale, the Locus of Control and Behavior Scale, the Visual Analogue Scale for fatigue, and the Profile of Mood Status were used to evaluate the severity of fatigue in the articles reviewed.
The level of chronic fatigue is related to different patterns of shifts. It is lowest among permanent day shift workers, followed by permanent night shift workers and those working multiple shifts except for the night shift. Severe chronic fatigue was reported in multiple shifts that included night shift (Samaha et al., 2007; Seki & Yamazaki, 2006; Winwood et al., 2006). Night shift nurses reported more severe fatigue than day or evening shift nurses. There are various possible explanations. In addition to needing to take care of more patients and usually going off duty later, many night shift nurses have sleep pattern disorders and shorter prework sleep time. Self-reported health conditions were worse among night shift staff than day shift staff.
Some studies have found that fixed shift workers adapt better and experience less fatigue than multiple shift staff (Edell-Gustafsson et al., 2002; Seki & Yamazaki, 2006; Takeyama et al., 2005) and that untreated acute fatigue may develop into chronic fatigue. Day shift and single shifts cause less chronic fatigue, and workers recover more easily from fatigue in comparison with night shift and multiple shift workers. The forward-rotating shift (i.e., day shift → evening shift → night shift → rest) produces more refreshing sleep and less fatigue than the backward-rotating shift (Brooks & Swailes, 2002; Hossain et al., 2004; Winwood et al., 2006).
Chronic fatigue affects work performance, and studies reviewed show that fatigue affects the worker similar to alcohol intoxication. Night shift workers getting up at 7 a.m. on the first day and working to the next night shift (about 24 hours of sleeplessness) have a state of consciousness similar to that of a person with a 0.1% blood alcohol concentration (Lamond et al., 2004). Workers experienced 30-second episodes spontaneous episodes of uncontrolled sleep, and the body was subject to "microsleeps" without any response to external stimuli (Lamond et al., 2004). Persistent fatigue and inefficient recovery are the main causes of critical incidents at work. Investigations of the frequency of critical incidents have shown that incidence was higher during the night shift as compared with day shift because of the behavioral compromise of shift workers at night (Smith et al., 1994).
Physical condition and the ability to adapt to circadian rhythm both decrease with increasing age. The ability to adapt to shift patterns relates negatively to age. People older than 40 years feel more fatigued and exhibit poorer adaptiveness. Night shifts and multiple shifts produce more severe fatigue and poor recovery. Therefore, people older than 50 years should avoid night shift work (Harma et al., 2002; Winwood et al., 2006).
Effect of Shift Work on Attention
Activity and sleep alternate between day and night. Night shift affects sleep quality and quantity. Poor sleep quality causes sleep deficits. Workers with sleep deficits are tired, have poor concentration, and do not function at peak behavioral levels, which, in turn, may lead to increased workplace risks. Studies have indicated that multiple shift work rotations result in poor adjustment, attention, concentration, and short-term memory as well as higher fatigue (Valdez et al., 2005).
Table 5 summarizes previous reports on the effect of shift work on attention level. According to the QSA, levels of evidence in these studies ranged from II to IV. Study subjects included nursing staff, physician residents, surgical trainee specialists, aircraft maintenance engineers, shift workers in various occupations, and college students. Attention assessment tools used included the Visual Analogue Scale (100-mm scale), the computerized unprepared simple reaction time task, the adjective checklists, the Pauli test, the numerical memory test, addition test, choice reaction time, memory scanning, picture recognition, the Mackworth clock vigilance task, the digit-symbol substitution subtest of the Wechsler Adult Intelligence Scale, and the selective attention test derived from the Sternberg test.
Sleep deprivation can affect short-term memory and performance speed; studies of neuropsychiatric function in shift workers or night shift workers have shown that attention and cognitive speed decreases in these subjects. Worker reaction times were prolonged, and the duration without response lasted more than 10 seconds. Furthermore, the speed of mathematical calculation and judgment also decreased. Electroencephalogram analyses demonstrate that, compared with day shift workers, night shift workers exhibit decreased alpha waves and increased beta and delta waves. These results indicate decreased attention and psychomotor activity among night shift workers. A nap before a scheduled shift can decrease delta waves, whereas taking a nap during the first night shift significantly improves response speed (Frey et al., 2002; Purnell et al., 2002).
There was no significant difference in cognition between evening shift and day shift workers. However, in comparison with regular shift workers, night shift workers had poorer cognitive performance. Subjects engaged in shift work over a period of 1 to 4 years found decreased memory performance. Memory performance was even worse in employees engaged in shift work for 10 to 20 years (Dingley, 1996; Frey et al., 2002; Lee et al., 2003; Rouch, Wild, Ansiau, & Marquie, 2005; Seki & Yamazaki, 2006; Valdez et al., 2005). Attention is related to fatigue level, with employee vigilance higher at the beginning of a shift than at the end. For more than 3 to 8 days of continuous night shifts, staff reaction time decreased by 3.37%, 0.99%, and 0.75% per hour during the first, fourth, and seventh night shift, respectively (Dingley, 1996).
Arranging shift schedules that extend the 24-hour biological clock is suggested because it facilitates better adaptation. Forward rotating is better than backward rotating the shift because the former is better suited to adjusting circadian rhythm patterns. Longer periods of rest, usually 1 to 2 days, are suggested between different shifts (Hossain et al., 2004; Scott, Rogers, Hwang, & Zhang, 2006). Evidence suggests that work duration should be less than 12 hours, and taking a short break during work to prevent work errors attributable to decreased vigilance is recommended. If shift rotation is inevitable, forward rotating is preferable.
According to our systematic literature review, there are various inconsistent measures for the same variable and only a few studies used an RCT design. Because of the paucity of studies and significant heterogeneity in study population, outcome events, and results, we synthesized data in a qualitative manner. Thus, we did not conduct meta-analysis.
Reviewed topics were deliberately broad, and identified studies highlighted different influences relating shift work to nursing staff circadian rhythm. From the literature reviewed, it is evident that shift work remains a complex problem that simultaneously affects multiple aspects, for example, sleep-awake cycle disturbances, circadian misalignments, and predisposing individual and domestic factors. A majority of studies focused on single specific factors affected by shift work, including circadian rhythm, sleep quality, fatigue, or attention. In the future, a comprehensive, multilevel approach is needed to remedy the lack of research on the simultaneous comparison of the effects of shift work on nursing staff physiology and mentality.
This review of existing literature suggests that constant rotation of duty may make maintaining a good working symbiosis more difficult, in terms of biochemistry, behavior, and psychology. For example, plasma concentrations of cortisol, body temperature, sleep-wake cycles, and moods all exhibit cyclical circadian rhythms. Cortisol profiles in night shift workers who sleep during the day are relatively high and melatonin secretion is inhibited by light, leading to poor sleep quality. Adjustment of circadian rhythm is slowed by shift work. Cortisol increases at night during Day 5 of continuous night shifts, and the value is higher in the afternoon than that in the morning, presenting a reverse circadian rhythm during night shift adjustment. These findings suggest that working in a fixed shift can better maintain circadian rhythm. If rotating shift work is necessary, the easiest and most adaptable way is forward shifts, moving from days to evenings to nights.
As previously stated, night shift workers should work at night and sleep during the day. When sleeping during the day, cortisol secretion increases, which diminishes the healing power of sleep. Furthermore, because of the fact that duration of daytime sleep is also generally shorter in comparison to sleeping at night, studies have found that if subjects go to sleep between 8:00 and 10:00 p.m., sleep duration would be relatively long. However, if subjects go to sleep between 10:00 a.m. and 12:00 p.m., sleep quality was worse and sleep duration the shortest. On the second day of the night shift, NREM sleep cycles usually decrease, REM sleep increases, and then total sleep duration decreases. Night shift workers sleep 1 to 4 hours less than daytime workers on average (average 2 hours). The shortening of daytime sleep in night shift workers affects sleep quality. Increasing daytime sleep duration on the first day of the night shift can decrease employee fatigue and, subsequently, increase sleep quality. An appropriate sleeping environment to avoid interference and maintain good sleep hygiene can improve night worker sleep quality.
Working multiple shifts (include night work) was associated with higher acute work-related fatigue, poorer recovery, and higher maladaptive chronic fatigue. Workers on night shift experienced more fatigue than on day and evening shifts. The present review indicated that with an increase in the number of workdays, level of fatigue also increases. Appropriate rest days between different shifts should be scheduled to allow workers time to recover from circadian rhythm disturbances. In addition, regular breaks and meal times during the work day can allow nurses to rest and recover from fatigue.
Night shift workers showed slower response speed and lower accuracy levels in comparison with day shift workers. When nurses work night shifts, lack of sleep and forced somnolence during the normal circadian rhythm sleeping phase may reduce alertness and compromise work safety and productivity. Sufficient rest can assist the individual to avoid errors in work. To avoid sleep debt accumulation, excessive workdays should not be arranged in succession. Overall, the fewest problems and the least amount of disruptions occur for staff who adhere to a regular work pattern. An evening shift lifestyle suits staff with average temperature rhythms who perform better in the afternoon and evening, whereas "night owls" may perform better when working nights. Studies show that the circadian rhythms of shift workers completely adjusted after approximately four to five consecutive duties, especially when transferring from day to night shift work. However, evidence also shows that injury risk for night shift staff rose at the end of the workweek. This finding seems inconsistent with any beneficial effect of circadian adjustment over successive night shifts. Rosters, therefore, must simultaneously consider that staff may suffer from fatigue because of long working hours (12 hours) as well as long consecutive stretches of night duty (weeks or months on end).
Nevertheless, managers who want to minimize adverse effects on shift workers should maintain fixed scheduling to decrease circadian rhythm disruptions. This review suggests several countermeasures for scheduling arrangements, such as forward-rotating shifts and scheduling a maximum of three consecutive night shifts, a minimum of two consecutive days off to rest sufficiently before returning to work, and taking regular short breaks during working hours when the scheduled shift is less than 12 hours long.
Undoubtedly, shift work interferes with worker circadian rhythms and, subsequently, affects sleep quality. Shift work also causes fatigue and decreases attention levels, which can result in decreased work performance and more critical incidents. Physiology and work efficacy are especially important to maintaining a healthy workforce. To develop a proper shift rotation schedule, administrators who prepare and coordinate nursing rosters should consider work patterns, circadian rhythm disorders caused by night shift and rotating shift work, incentives for working night shifts, and workplace characteristics. As most employees cannot rapidly adjust their circadian rhythm to match changing working hours, we suggest that employees keep to regular shifts and working hours as much as possible to better maintain regular circadian rhythm. If shift work is necessary, shifting "forward" is the easiest way to allow the body to adapt. Extended working hours should be avoided because of negative impacts on attention and physical strength. Regular short breaks during working hours can improve work performance. A proper shift rotation system and staff education on sleep hygiene and knowledge of circadian rhythm functions can increase staff retention and improve workplace morale.
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