Several functions in the human body exhibit circadian periodicity, such as core temperature, hormones such as cortisol, melatonin, testosterone, and growth hormone. In addition to these, the sleep-wake cycle exhibits circadian periodicity. The latter is regulated by two processes: circadian process (C) and homeostatic process (S). Process C refers to the body's circadian regulation, while process S refers to the accumulation of fatigue throughout the day. Numerous factors regulate our endogenous rhythms, the main one being sunlight. Thus, the human body has evolved over time to remain active during the day, while there is sunlight, and to rest during the night.
However, we live in a non-stop society, so it is called a 24/7 society (working 24 hours a day, 7 days a week). Thus, some professions and operations need to carry out their activities during the night period, such as police, firefighters, doctors, nurses, miners, and industrial operations. Night work is defined as work that starts between 10 pm and 5 am, while shift work occurs at different times of the day (morning, afternoon, and night), occurring on a rotating basis. Worldwide, approximately 25% of workers are involved in some shift work routine. In Brazil, it is estimated that 15% of workers are inserted in some shift work routine (COSTA; FISCHER; MORENO; ROTENBERG, 2004)COSTA; FISCHER; MORENO; ROTENBERG, 2004)
Scientific evidence demonstrates that this population often suffers from circadian misalignment and sleep restriction, and this reflects negatively on the health of these workers. Sleep disturbances, cardiometabolic disorders, psychological changes and cancer are some problems generated by working in unconventional hours. Next, the health consequences of night and shift work will be discussed, and the possible causes of these issues.
Relation between night and shift work and sleep
Shift workers frequently have total sleep time below the recommended parameters for the population (over 7 hours per light-dark cycle). A meta-analysis conducted in 2000 identified that workers sleep, on average, 5h50min after night shifts, and 8h02min after afternoon shiftsPILCHER; LAMBERT; HUFFCUTT, 2000On the other hand, it was identified that workers sleep on average 6h37min before morning shifts (AKERSTEDT, 2003).AKERSTEDT, 2003These findings can be explained by circadian factors. During night shifts, workers perform their activities at the circadian time of the greatest propensity to sleep and, consequently, need to rest at the time less prone to sleep (daytime). Thus, factors such as high core temperature, increased cortisol concentration and reduced melatonin concentration are factors that negatively impact the sleep of workers who need to sleep during the day. On the other hand, workers working in morning shifts, especially with a very early start (between 5 and 7 am), need to wake up at dawn. In this case, sleep restriction may also be caused by the inability to advance sleep schedules. Considering the characteristics of the endogenous rhythm (approximately 24.5 h), it is easier to delay sleep onset than the reverse. Another factor that contributes to sleep restriction in shift workers is the interval between shifts. There is evidence that very short intervals between shifts (less than 11 hours) dramatically influence the total sleep time of workersVEDAA; HARRIS; BJORVATN; WAAGE et al., 2016Furthermore, very long working hours also negatively influence the sleep time of workers by allowing little free time for leisure, housework, and rest.
Health consequences related to sleep restriction
Sleep is responsible for maintaining fundamental functions in the body, such as immune regulation, hormonal and metabolic balance, reducing inflammatory patterns and promoting individual well-being. Thus, sleep restriction causes several harms to health. Below we will see some consequences of this condition.
A systematic review and meta-analysis conducted in 2012 demonstrated that shift workers are more prone to myocardial infarction and ischemic strokeVYAS; GARG; IANSAVICHUS; COSTELLA et al., 2012Analysis of various work shifts showed that the shift most prone to cardiovascular disease was the night shift, while the afternoon shift was not related to cardiovascular disease.
Two systematic reviews concluded that night work increases the risk for weight gain and obesityPROPER; VAN DE LANGENBERG; RODENBURG; VERMEULEN et al., 2016; VAN DRONGELEN; BOOT; MERKUS; SMID et al., 2011Also, there is evidence that shift workers are more likely to develop type 2 diabetes.
There are studies showing that the evidence is insufficient to determine whether there is in fact a relation between shift work and cancer, while other studies have concluded that long exposures (greater than 20 years) increase the risk of developing cancer. However, there is evidence for prostate cancerRAO; YU; BAI; ZHENG et al., 2015) e câncer colorretal (WANG; JI; ZHU; LIANG et al., 2015).
Accidents in the workplace
Shift workers are at increased risk for accidents, both inside and outside the work environment. Circadian factors contribute to this scenario, as human performance is directly correlated with core temperature. Thus, during the night, the core temperature is at its nadir (the lowest value found during the day) and, consequently, performance is reduced (longer reaction time, worse perception of the environment and worse ability to make decisions), thus increasing the risk for accidents.
In addition, factors inherent to the work schedule such as shift length and successive shifts interfere in this aspect. Evidence demonstrates that the night shift presents a greater risk for accidents, and that this risk increases as successive shifts accumulate. In addition, the risk of accidents is greater the longer the shift lasts, with evidence showing that shifts over 12 hours have twice the risk when compared to shifts of 6 hoursFOLKARD; TUCKER, 2003TUCKER, 2003). To learn more, read https://www.condorinst.com.br/fadiga-e-sono/.
In summary, evidence points to an increased risk of cardiovascular and metabolic diseases and cancer for shift workers, specifically for those who work the night shift. The similarity of results related to night work and insufficient sleep suggest that insufficient total sleep time and unsatisfactory quality are responsible for the diseases and accidents observed in shift workers.
Mechanisms responsible for the adversities experienced by shift workers
The factors responsible for the problems experienced by workers can be grouped into behavioral, psychosocial stress, and physiological mechanisms.
Shift work is responsible for changing several behavioral factors, such as sleep patterns, exposure to light, eating patterns, level of physical activity, alcohol, and tobacco intake. When staying awake at night, there is a reduction in the secretion of melatonin, a hormone responsible for various functions in the body and, among them, immune functionCIPOLLA-NETO; AMARAL, 2018In fact, the reduction in melatonin secretion is one of the mechanisms proposed for a greater risk of developing cancer in shift workers, as it has an antitumor component. A meta-analysis conducted in 2015 concluded that long weekly working hours (over 48 hours) are related to higher alcohol intakeVIRTANEN; JOKELA; NYBERG; MADSEN et al., 2015Not only that, there is evidence showing that shift workers report smoking more frequentlyHÄRMÄ, 2006and exercise less frequentlyVANDELANOTTE; SHORT; ROCKLOFF; DI MILLIA et al., 2015).
Working at unconventional hours causes an imbalance between work and family demands. Generally, family meetings and meetings with friends take place at times when workers need to rest or are exercising their work activities. Thus, the imbalance between personal life and work is associated with a poor perception of healthBAMBRA; WHITEHEAD; SOWDEN; AKERS et al., 2008).
Studies show that changing eating times, sleeping patterns and light exposure alters the secretion of regulatory hormones in the body, such as testosterone, growth hormone, melatonin, cortisol, insulin, and prolactin.
The altered sleep pattern causes disturbances in perceived stress, generating an increase in the stressful response. Evidence indicates that shift workers have a greater response to stress, as well as greater activation of the sympathetic nervous systemFARAUT; BAYON; LÉGER, 2013).
Similarly, the altered sleep pattern negatively impacts glucose utilization and insulin sensitivity, altering the body's energy balance. Working in shifts and especially during the night shift facilitates food intake during periods outside the organism's phase, and there is evidence showing that eating at night is related to worse lipid metabolismRIBEIRO; HAMPTON; MORGAN; DEACON et al., 1998). In addition, food preference during the night period is characterized by foods of higher value. Caloric foods such as snacks and fatty foods over healthy foods such as salads and fruits. This is due to the reduction in core temperature due to circadian factors, which leads to the search for high-calorie foods, as well as the dysregulation between the hormones leptin and ghrelin (increase in ghrelin, the hormone responsible for the sensation of hunger, and reduction in leptin, hormone responsible for the feeling of satiety).
Studies indicate that poor sleep quality as well as insufficient total sleep time is related to arterial hypertensionGUO; ZHENG; WANG; ZHANG et al., 2013Furthermore, there is also a relation between shift work, worse lipid profile, and blood clotting. Finally, there is evidence showing that shift workers are at increased risk for the development of subclinical atherosclerosis, and this condition presents itself even in workers under 40 years of agePUTTONEN; KIVIMÄKI; ELOVAINIO; PULKKI-RÅBACK et al., 2009).
Altered immune function
Altered sleep pattern is related to higher levels of inflammatory markers such as C-reactive protein and interleukinIRWIN, 2015It is well established that sleep aids in the T helper 1 cell response and sleep restriction leads to T helper 2, and this reaction is similar to other types of stress. Thus, sleep restriction and poor quality sleep are associated with a higher probability of getting the common flu. Still, Silva et al. expose that shift workers are more exposed to contracting the COVID-19 virus, and this fact is due to factors such as altered sleep pattern, sedentary behavior and circadian misalignmentSILVA; GUERREIRO, 2020).
Individual tolerance to shift and night work
Some people tolerate shift work and night work better than othersSAKSVIK; BJORVATN; HETLAND; SANDAL et al., 2011Differences in genes related to circadian and homeostatic regulation lead to differences in sleep pressure, fatigue, and performanceARENDT, 2010Thus, evidence indicates that workers with the afternoon chronotype seem to adapt better to shift work and night work, as well as those who are able to sleep at different times during the day. Not only, personality aspects such as neuroticism, low extraversion, low testosterone and the need to sleep long hours seem to be related to poor adaptation to shift work. Therefore, these people become a risk group for night work.
How to Assess the Sleep of Workers
Evidence shows that insufficient sleep and an unsatisfactory amount is one of the main factors that harm workers' health. Thus, it is essential to assess the quality of sleep in this population. In this regard, actigraphy is a useful and non-invasive technique that can be used to assess sleep and biological rhythms (Sadeh, 2011). The actigraph is a piece of equipment similar to a wristwatch and contains sensors for movement, light and, in some more modern models, temperature sensors. Data analysis allows obtaining variables such as total sleep time, sleep efficiency (relation between time in bed and time the individual actually slept), latency to sleep onset (time elapsed between the intention to sleep and sleep onset), time awake after sleep onset, and total waking time. For more information, see https://condorinst.com.br/o-que-e-actigrafia/.
Night and shift workers are exposed to conditions that substantially affect their health and quality of life. Circadian misalignment and poor quality sleep in insufficient quantity lead to immunological, cognitive and cardiometabolic alterations. In addition, psychosocial stress resulting from unsynchronized schedules with family and friends, as well as sedentary habits and inappropriate eating behavior contribute to this negative scenario. Thus, it is important that this population is regularly evaluated in order to identify possible sleep disorders, as well as the adaptation of the work scale into a model that better fits the workers' profiles. In this aspect, it is important to consider conditions such as the duration of the working day, interval between shifts that must be less than 11 hours, the speed, and direction of rotation of shifts, as well as the number of successive night shifts, breaks between and within shifts, and creating strategies for creating a welcoming and friendly work environment.