Shift work sleep disorder (SWSD) is a circadian rhythm sleep disorder characterized by insomnia, excessive sleepiness, or both affecting people whose work hours overlap with the typical sleep period. Insomnia can be the difficulty to fall asleep or waking up before the individual has slept enough. About 20% of the working population participates in shift work. SWSD commonly goes undiagnosed, and it is estimated that 10–40% of shift workers have SWSD. The excessive sleepiness appears when the individual has to be productive, awake and alert. Both symptoms are predominant in SWSD. There are numerous shift work schedules, and they may be permanent, intermittent, or rotating; consequently, the manifestations of SWSD are quite variable. Most people with different schedules than the ordinary one (from 8 AM to 6 PM) might have these symptoms but the difference is that SWSD is continual, long-term, and starts to interfere with the individual's life.
Health effects There have been many studies suggesting health risks associated with shift work. Many studies have associated sleep disorders with decreased bone mineral density (BMD) and risk for fracture. Researchers have found that those who work long-term in night positions, like nurses, are at a great risk for wrist and hip fractures (RR=1.37). Low fertility and issues during pregnancy are increased in shift workers. Obesity, diabetes, insulin resistance, elevated body fat levels and dyslipidemias were shown to be much higher in those who work night shift. SWSD can increase the risk of mental disorders. Specifically, depression, anxiety, and alcohol use disorder is increased in shift workers. Because the circadian system regulates the rate of chemical substances in the body, when it is impaired, several consequences are possible. Acute sleep loss has been shown to increase the levels of t-tau in blood plasma, which may explain the neurocognitive effects of sleep loss.
Sleep quality Sleep loss and decreased quality of sleep is another effect of shift work. To promote a healthy lifestyle, the American Academy of Sleep Medicine recommended that an adult have 7 or more hours of sleep per day. Sleep deprivation and sleep disorders are factors that contribute significantly to these errors. In the same article, the authors affirm that there is a high prevalence of sleepiness and symptoms of sleep disorders related to the circadian system in medical center nurses. In a study done with around 1100 nurses, almost half of them (49%) reported sleeping less than 7 hours per day, a significant increase compared to national figures, in which 28% of people claimed to sleep less than 7 hours per night. Having a lack of sleep can impact cognitive performance. For example, it might become difficult to stay focused and concentrate, and reaction times might also be slowed down. SWSD might interfere with making decisions quickly, driving, or flying safely. Sleep loss seen in shift workers greatly impairs cognitive performance; being awake for 24 hours straight results in a cognitive performance that is equal to a blood-alcohol of 0.10, which is over the legal limit in most states. All of these factors can affect work efficiency and cause accidents. Michael Lee et al. demonstrated that those working night shifts had a significantly higher risk of hazardous driving events when compared to those on a typical day shift schedule. Accidents in the workplace have been found to be 60% higher in shift workers. They can affect the individual's social life and cause a lack of well-being and happiness. Poor sleep quality has also been associated with decreased quality of life, based on a SF-36 assessment.
Sleep and alertness Although SWD affects many shift workers, its manifestation is still unclear within the general shift-working population. A field study investigating the nature of SWD in an experimental (with SWD) and control (non-SWD) group of Finnish shift workers revealed decreased total sleep time (TST) and increased sleep deficit before morning shifts. Furthermore, the SWD group also exhibited decreased objective sleep efficiency, decrease in sleep compensation over the free days, increased sleep latency, and finally poorer sleep quality was recorded in the SWD group compared to the non-SWD group. Moreover, shift workers with SWD scored significantly higher on the Karolinska Sleepiness Scale (KSS) when assessed at the beginning and the end of morning shifts and at the end of night shifts, while having more attentional lapses at the beginning of night shifts. Many studies have shown evidence of how partial and total sleep deprivation affects work productivity, absenteeism, fatal workplace accidents, and more. In a study by Akkerstedt et al., those who had a hard time sleeping in the past two weeks were at a greater risk for having a fatal workplace accident (RR=1.89, 95% CI 1.22–2.94). Other sleep disorders, like OSA which are risk factors for SWSD, have also been associated with low productivity, absenteeism, and accidents. At a cognitive level, sleep deprivation has been shown to cause decreased attentiveness, increased micro-sleeps, delayed psychomotor response, performance deterioration, neglect of activities, decline in working memory, and more.
Immune functioning Partial and total sleep deprivation has been linked to an increase of pro-inflammatory markers, such as IL-6, and a decrease in anti-inflammatory markers, such as IL-10, that plays a role in tumor suppression. Chronic shift work has been associated with decreased immune function in nurses. In a study by Naigi, et al., over the course of a shift, nurses exhibited decreasing levels of Natural Killer cells, an innate immune response that plays a role in infectious disease and tumor suppression. Other researchers have found that less sleep at night increased the risk of developing the common cold. A supporting study by Moher et al. showed that shift workers were more likely to develop infectious diseases after exposure compared to daytime workers. A poorly functioning immune system may leave workers vulnerable for developing occupational illnesses. Sleep loss is also associated with an increase in TNF, a marker of systemic immune functioning.
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