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Shift Work: Disrupted Circadian Rhythms and Sleep - Implications for Health and Well-Being

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Purpose of review: Our 24/7 society is dependent on shift work, despite mounting evidence for negative health outcomes from sleep displacement due to shift work. This paper reviews short- and long-term health consequences of sleep displacement and circadian misalignment due to shift work. Recent findings: We focus on four broad health domains: metabolic health; risk of cancer; cardiovascular health; and mental health. Circadian misalignment affects these domains by inducing sleep deficiency, sympathovagal and hormonal imbalance, inflammation, impaired glucose metabolism, and dysregulated cell cycles. This leads to a range of medical conditions, including obesity, metabolic syndrome, type II diabetes, gastrointestinal dysfunction, compromised immune function, cardiovascular disease, excessive sleepiness, mood and social disorders, and increased cancer risk. Summary: Interactions of biological disturbances with behavioral and societal factors shape the effects of shift work on health and well-being. Research is needed to better understand the underlying mechanisms and drive the development of countermeasures. The final publication is available at
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Shift Work: Disrupted Circadian Rhythms
and SleepImplications for Health and Well-being
Stephen M. James
&Kimberly A. Honn
&Shobhan Gaddameedhi
Hans P.A. Van Dongen
Published online: 27 April 2017
#Springer International Publishing AG 2017
Purpose of Review Our 24/7 society is dependent on shift
work, despite mounting evidence for negative health out-
comes from sleep displacement due to shift work. This paper
reviews short- and long-term health consequences of sleep
displacement and circadian misalignment due to shift work.
Recent Findings We focus on four broad health domains: met-
abolic health, risk of cancer, cardiovascular health, and mental
health. Circadian misalignment affects these domains by in-
ducing sleep deficiency, sympathovagal and hormonal imbal-
ance, inflammation, impaired glucose metabolism, and dys-
regulated cell cycles. This leads to a range of medical condi-
tions, including obesity, metabolic syndrome, typeII diabetes,
gastrointestinal dysfunction, compromised immune function,
cardiovascular disease, excessive sleepiness, mood and social
disorders, and increased cancer risk.
Summary Interactions of biological disturbances with behav-
ioral and societal factors shape the effects of shift work on
health and well-being. Research is needed to better understand
the underlying mechanisms and drive the development of
Keywords Circadian misalignment .Sleep displacement .
Metabolic health .Cancer risk .Heart health .Mental health
It has long been recognized that shift work has a negative
impact on health and well-being. Historically, this has been
attributed to adverse effects of long work hours, nighttime
light exposure, and psychosocial factorseffects that are still
recognized as relevant for tolerance to shift work [1,2].
However, the health consequences of shift work should first
be understood in terms of a fundamental misalignment be-
tween the circadian (i.e., near-24-h) rhythmof the endogenous
biological clock and the timing of the sleep/wake cycle [3].
While this paper is concerned primarily with the long-term
health consequences of shift work, the implications of circa-
dian misalignment between the biological clock and the sleep/
wake cycle are perhaps best illustrated by how such misalign-
ment increases the risk of workplace accidents and injuries.
In healthy, non-shift workers with normal sleep patterns,
daytime wakefulness is driven by the biological clock, which
produces circadian rhythmicity driving increased alertness
during the daytime and decreased alertness during the night-
time [4,5]. This circadian process is counteracted by a ho-
meostatic pressure for sleep, which builds across waking
hours [6,7]. When working during daytime hours, these two
processes function in concert and in synchrony with the envi-
ronmental light/dark cycle to maintain alertness while awake
and at work, while allowing for consolidated sleep during the
night. Working nights or early morning shifts means that an
individual must be awake when the circadian drive for alert-
ness is low and asleep when it is high, in opposition to the
natural biological rhythm. This leads to shortened and
disrupted sleep and excessive sleepiness while awake [8,9].
This article is part of the Topical Collection on Circadian Rhythm
*Hans P.A. Van Dongen
Sleep and Performance Research Center, Washington State
University, Spokane, WA, USA
Elson S. Floyd College of Medicine, Washington State University,
Spokane, WA, USA
College of Pharmacy, Washington State University, Spokane, WA,
Curr Sleep Medicine Rep (2017) 3:104112
DOI 10.1007/s40675-017-0071-6
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... There are possibilities to prevent the health effects of shift work, but it is impossible to eliminate the negative effects of night work. People responsible for the organization of shift work should take steps to improve working conditions and plan the time of professional activity so that it is adapted to the psychophysical abilities of the employee [11]. The Directive of the European Parliament and of the Council on certain aspects of the organization of working time, states that the situation of night and shift workers requires that the level of protection of their safety and health should be adapted to the nature of the work [12]. ...
... Work ergonomics and health education of employers and employees regarding potential health hazards at work with excessive night shifts are still underestimated and requiring attention in areas of health care institutions. Proper medical care is also important for maintaining the health of the employee [11]. ...
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Introduction: Nursing staff working in a shift or night system are exposed to sleep disorders , which has a direct impact on the emergence of dangerous health consequences for them. Mel-atonin secretion is abnormal at night and the circadian rhythm is disturbed. The aim of the study was to assess the occurrence of sleep disorders and their consequences for the body in a group of representative nursing staff working in a shift and night system. Participants: The study was conducted among 126 nurses who are generally healthy, employed in health care facilities in the Mało-polskie voivodship. Methods: The Athens Insomnia Scale consisting of 8 test items was used to obtain research material: falling asleep, waking up at night, waking up in the morning, total sleep time, sleep quality, well-being the next day, mental and physical fitness the next day, and sleepiness during the next day. As well as an original questionnaire. Results: The research showed significant negative consequences of shift work on the health of health-care workers. The subjects noticed symptoms related to the nervous system, such as increased nervous tension 53%, lack of patience in 62% of all respondents. As many as 85% pointed to the negative impact of shift work on their family life, 82% of all respondents on social life and 56% of all respondents on sex life. The other variables were not confirmed. Conclusions: Symptoms of insomnia are common among night-work nurses.
... Today, 15-20% of the working population is on duty outside of traditional daytime hours. 1 Shift workers regularly experience sleep disturbances attributable to desynchronization of the homeostatic sleep pressure and circadian rhythm; thus, they are predisposed to the development of shift work disorder. 2 A recent meta-analysis found that 26.5% of all shift workers met the diagnostic criteria for this disorder, thus severe disturbances during sleep and/or excessive sleepiness during waking. 3 Several previous studies have described structural and functional brain changes in shift workers. ...
... Cell lines, characteristics, and maintenance (pg. 48) placing them at high risk for both long and short-term consequences of constant circadian misalignment and disruptions resulting in systemic consequences such as increased disruptions to metabolic health, propensity towards certain cancers, negative impacts on mental well-being and heart problems compared to their non-shift work counterparts (James, 2017). These negative effects on our overall health are well beyond the scope of what is given in this thesis and we have yet to gain a full understanding of the depth and breadth of the consequences we now face. ...
Maintaining our internal circadian (i.e. 24 -hour) clock is imperative to our daily biological and mental well-being. Large epidemiological studies have shown that disruptions of our circadian rhythms can lead to poor mental health, metabolic diseases, and various types of cancer. Various external cues that have become a part of the modern times such as electricity, shift -work, rapid travel across various time zones, easier access to nutritionally unbalanced food items, and various rigid social demands have deleterious effects on our internal clock, and generally reduce robustness of the circadian clock. The two following projects aim to examine two fundamental aspects of the circadian clock mechanism in health and disease: 1) exercise as an entrainment mechanism for the clock; and 2) targeting of clock proteins to reduce proliferation and survival in acute myeloid leukemia. Both projects center on the important role of the circadian clock protein brain and muscle ARNT-like factor (BMAL1), which is a transcription factor critical for maintaining cellular rhythmicity. Loss of the protein in the PVN of the hypothalamus can lead to arrhythmicity, but can be rescued by exercise. In the case of acute myeloid leukemia, this protein can be targeted to slow the proliferation of transformed cells. vii Together, these data suggest that the exploitation of circadian mechanisms of entrainment and molecular targeting can be used to rescue the damage inflicted by circadian disruption.
... Thus, working in shifts in the industry will add another level of overwhelming pressure and challenge for an individual. For those with standard sleep patterns, daytime wakefulness is driven by the biological clock, which produces circadian rhythmicity, driving increased alertness during the daytime and decreased attention during the night [1,35]. Shift workers involved in night shifts are reported to be more susceptible to psychological and mental health issues than those working regular hours. ...
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Background: The recent pandemic of COVID-19 has had a tremendous impact on healthcare frontliners. This study sought to assess healthcare shift workers' depression, anxiety, and stress and its associated factors. Methods: The sampling frame includes healthcare shift workers directly managing COVID-19 cases around Klang Valley, Malaysia. The participants' mental health status was assessed using the Depression, Anxiety, Stress Scale-21 (DASS-21). The associated factors specified in this study include sleep quality, physical activities, and eating habits. Pearson's χ2 and simple and multivariable binary logistic regression models were constructed following the Hosmer-Lemeshow approach to determine the potential associated factors. Results: A total of 413 participants were recruited. Overall, 40.7% of participants had one or more symptoms of depression, anxiety, or stress. Poor sleep quality was significantly associated with all mental health outcomes of depression, anxiety, and stress. Inactivity was found to be strongly associated with symptoms of depression and anxiety. At the same time, eating habits were strongly associated with anxiety and stress. Conclusions: Sleep quality, inactivity, and eating habits that were found to be associated with the mental health status of healthcare shift workers are modifiable factors that must be addressed to curb mental health issues among this group of workers.
... Inconsistencies in the timing of Zeitgeber exposure can also produce misalignment because the entrainment of the core clock to nutrient signaling occurs at different rates in peripheral tissues (Damiola et al., 2000). This is important because circadian clock misalignment is strongly associated with metabolic diseases (James et al., 2017;Kervezee et al., 2020;Kolbe et al., 2019;Scheer et al., 2009). On the other hand, improving circadian alignment through time-restricted feeding protects metabolic health (Chaix et al., , 2021Jamshed et al., 2019). ...
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The circadian clock regulates metabolism in anticipation of regular changes in the environment. It is found throughout the body, including in key metabolic organs such as the liver, adipose tissues, and intestine, where the timing of the clock is set largely by nutrient signaling. However, the circadian clocks of these tissues during the fasted state have not been completely characterized. Moreover, the sufficiency of a functioning host clock to produce diurnal rhythms in the composition of the microbiome in fasted animals has not been explored. To this end, mice were fasted 24 h prior to collection of key metabolic tissues and fecal samples for the analysis of circadian clock gene expression and microbiome composition. Rhythm characteristics were determined using CircaCompare software. We identify tissue‐specific changes to circadian clock rhythms upon fasting, particularly in the brown adipose tissue, and for the first time demonstrate the rhythmicity of the microbiome in fasted animals. Fasting causes tissue‐dependent changes in core clock gene expression, with the core clock of BAT being most sensitive to fasting. Variation in the microbiome depends more on circadian time than on feeding or fasting condition.
Sleep deprivation (SD) causes significant deficits in multiple aspects of cognition, including sustained attention and working memory. Investigating the neural processes underpinning these cognitive losses has proven challenging due to the confounds of current animal tasks; many employ appetitive or aversive stimuli to motivate behavior, while others lack task complexity that translates to human studies of executive function. We established the Lux Actuating Search Task (LAST) to circumvent these issues. The LAST is performed in a circular, open-field arena that requires rats to find an unmarked, quasi-randomly positioned target. Constant low-level floor vibrations motivate ambulation, while light intensity (determined by the rodent's proximity to the target destination) provides continuous visual feedback. The task has two paradigms that differ based on the relationship between the light intensity and target proximity: the Low Lux Target (LLT) paradigm and the High Lux Target paradigm (HLT). In this study, on days 1–6, the rats completed nine trials per day on one of the two paradigms. On day 7, the rats were either sleep deprived by gentle handling or were left undisturbed before undertaking the opposite (reversal) paradigm on days 7–9. Our results showed that SD significantly impeded the ability of Long Evans rats to learn the reversal paradigm, as indicated by increased times to target and increased failure percentages compared to rats whose sleep was undisturbed. Rats also showed reduced learning with the HLT paradigm, as the initial task or as the reversal task, likely due to the rodents' photophobia limiting their motivation to navigate toward a bright light, which is required to succeed.
Background This study aimed to determine and define the elements of an Fatigue Risk Management System (FRMS) diagnostic tool to assist an organization in systematically assessing its level of implementation of an FRMS. Methods A modified Delphi process was used involving 16 participants with expertise in sleep science, chronobiology, and fatigue risk management within occupational settings. The study was undertaken in two stages 1) review of elements and definitions; 2) review of statements for each element. Each stage involved an iterative process, and a consensus rule of ≥60% was applied to arrive at a final list of elements, definitions, and statements. Results Stage 1: a review of elements (n=12) and definitions resulted in a final list of 14 elements and definitions with a consensus of ≥60% achieved after 2 Delphi rounds. Stage 2: a review of statements (n=131) resulted in a final list of 119 statements with a consensus of ≥60% achieved after 2 Delphi rounds. Conclusion The final FRMS diagnostic tool will enable an organization to systematically assess the level of implementation of their current FRMS and identify gaps and opportunities to reduce risk.
Scientists in sleep and circadian rhythms, public health experts, healthcare providers, partners, and stakeholders convened in 2020 for a 2-day meeting organized by the Canadian Sleep and Circadian Network to develop a national strategy for the integration of sleep and circadian rhythms into public health and policies in Canada. The objective of this paper is to present the national strategy that emerged from this meeting of 60 participants from across Canada. The meeting focused on 4 key target priorities: (1) atypical working schedules, (2) sleep and circadian rhythms of children and adolescents, (3) insomnia, and (4) impact of sleep apnea on health. Following constructive discussions over 2 days, it was decided that the following 4 strategic objectives should be prioritized to accelerate the integration of sleep and circadian rhythms into public health policies in Canada: (1) Increase public health sleep and circadian rhythm research, (2) Increase public health education and knowledge mobilization on sleep, (3) Inform and support public health sleep interventions and policies, and (4) Promote sleep health training. The participants recommended that research and public health efforts should address the needs along the continuum of sleep health. The committee noted that strategies and interventions could differ across contexts, settings, sectors, and jurisdictions. The national strategy also identified high-priority research questions in public health and recommended mechanisms to build research capacity, providing a path forward for the integration of sleep and circadian rhythms into public health research and policies.
Rotating shift work places a serious burden on nurses’ physical and psychological health. Gastrointestinal (GI) symptoms are a common complaint among shift workers. This study assessed GI symptoms and identified the associations between dietary habits, psychological status, and sleep quality among rotating shift nurses. Data from 125 female nurses in rotating shifts who worked at two tertiary hospitals in South Korea were collected using a questionnaire that included the Gastrointestinal Symptoms Questionnaire; the Dietary Habit Questionnaire; the Depression, Anxiety, Stress Scale (DASS)-21; and the Pittsburgh Sleep Quality Index (PSQI). All participants experienced various GI symptoms, and 47% of them complained of at least one severe GI symptom. There were significant differences in GI symptom scores according to the status of depression, anxiety, stress, and sleep quality. In multiple linear regression analysis, the factors associated with an increase in the occurrence and severity of GI symptoms were poor sleep quality and morbid anxiety and stress. The model explained power at 43.2%. As most nurses in rotating shifts experience GI symptoms, they should receive counseling and training programs at work to alleviate psychological symptoms, improve sleep quality, and pay more attention to their health status as well as GI symptom management.
Background Nurses play a pivotal role in promoting health for cancer prevention. Comparatively little is known, however, of their health-promoting behaviours and perceived lifestyle-related cancer risk factors. Aim To assess nurses’ health-promoting behaviours and perception of lifestyle-related cancer risk factors. Methods This is a descriptive, cross-sectional design study of 357 nurses from a teaching hospital. Respondents completed the Health-Promoting Lifestyle Profile II questionnaire. Perception of cancer risk factors was measured based on 29 well-established lifestyle factors. Findings Almost half of all nurses were overweight or obese (mean BMI = 25.2, SD = 4.95). The highest health-promoting behaviour mean score was for the spiritual growth subscale, while the lowest mean score was in physical activity subscale. Lifestyle-related cancer risk factors such as overweight/ obesity, practising diets high in red meat or diets low in vegetables/ fruit, and insufficient physical activities were not prioritised by the nurses. Conclusions Nurses in this sample were found to not engage in physical activity. A high proportion of nurses in this study attributed cancer risk to environmental rather than personal factors. The findings of the study enlighten nurse administrators in developing healthy lifestyle programs for nurses.
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Background It has been proposed that night shift work could increase breast cancer incidence. A 2007 World Health Organization review concluded, mainly from animal evidence, that shift work involving circadian disruption is probably carcinogenic to humans. We therefore aimed to generate prospective epidemiological evidence on night shift work and breast cancer incidence. Methods Overall, 522 246 Million Women Study, 22 559 EPIC-Oxford, and 251 045 UK Biobank participants answered questions on shift work and were followed for incident cancer. Cox regression yielded multivariable-adjusted breast cancer incidence rate ratios (RRs) and 95% confidence intervals (CIs) for night shift work vs no night shift work, and likelihood ratio tests for interaction were used to assess heterogeneity. Our meta-analyses combined these and relative risks from the seven previously published prospective studies (1.4 million women in total), using inverse-variance weighted averages of the study-specific log RRs. Results In the Million Women Study, EPIC-Oxford, and UK Biobank, respectively, 673, 28, and 67 women who reported night shift work developed breast cancer, and the RRs for any vs no night shift work were 1.00 (95% CI = 0.92 to 1.08), 1.07 (95% CI = 0.71 to 1.62), and 0.78 (95% CI = 0.61 to 1.00). In the Million Women Study, the RR for 20 or more years of night shift work was 1.00 (95% CI = 0.81 to 1.23), with no statistically significant heterogeneity by sleep patterns or breast cancer risk factors. Our meta-analysis of all 10 prospective studies included 4660 breast cancers in women reporting night shift work; compared with other women, the combined relative risks were 0.99 (95% CI = 0.95 to 1.03) for any night shift work, 1.01 (95% CI = 0.93 to 1.10) for 20 or more years of night shift work, and 1.00 (95% CI = 0.87 to 1.14) for 30 or more years. Conclusions The totality of the prospective evidence shows that night shift work, including long-term shift work, has little or no effect on breast cancer incidence.
Chronic jet lag induces spontaneous hepatocellular carcinoma (HCC) in wild-type mice following a mechanism very similar to that observed in obese humans. The process initiates with non-alcoholic fatty liver disease (NAFLD) that progresses to steatohepatitis and fibrosis before HCC detection. This pathophysiological pathway is driven by jet-lag-induced genome-wide gene deregulation and global liver metabolic dysfunction, with nuclear receptor-controlled cholesterol/bile acid and xenobiotic metabolism among the top deregulated pathways. Ablation of farnesoid X receptor dramatically increases enterohepatic bile acid levels and jet-lag-induced HCC, while loss of constitutive androstane receptor (CAR), a well-known liver tumor promoter that mediates toxic bile acid signaling, inhibits NAFLD-induced hepatocarcinogenesis. Circadian disruption activates CAR by promoting cholestasis, peripheral clock disruption, and sympathetic dysfunction.
What is the topic of this review? This report looks at the role of endothelial nitric oxide signalling in the time-of-day variation in vasoconstriction of resistance vessels. What advances does it highlight? It highlights a time-of-day variation in contraction of mesenteric arteries, characterized by a reduced contractile response to either phenylephrine or high K⁺ and increased relaxation in response to acetylcholine during the active period. This time-of-day variation in contraction results from a difference in endothelial nitric oxide synthase (eNOS) signalling that correlates with levels of eNOS expression, which peak during the active period and may have far reaching physiological consequences beyond regulation of blood pressure. There is a strong time-of-day variation in the vasoconstriction in response to sympathetic stimulation that may contribute to the time-of-day variation in blood pressure, which is characterized by a dip in blood pressure during the individual's rest period when sympathetic activity is low. Vasoconstriction is known to be regulated tightly by nitric oxide signalling from the endothelial cells, so we have looked at the effect of time-of-day on levels of endothelial nitric oxide synthase (eNOS) and vascular contractility. Mesenteric arteries isolated from the nocturnal rat exhibit a time-of-day variation in their contractile response to α1-adrenoreceptor and muscarinic activation, which is characterized by a reduced vasoconstriction in response to phenylephrine and enhanced vasodilatation in response to acetylcholine during the rat's active period at night. An increase in eNOS signalling during the active period is responsible for this time-of-day difference in response to phenylephrine and acetylcholine and correlates with the large increase in eNOS expression (mRNA and protein) during the active period, possibly driven by the presence of a functioning peripheral circadian clock. This increase in eNOS signalling may function to limit the increase in peripheral resistance and therefore blood pressure during the increased sympathetic activity. © 2016 The Authors. Experimental Physiology
Circadian rhythms are 24-hr oscillations that control a variety of biological processes in living systems, including two hallmarks of cancer, cell division and metabolism. Circadian rhythm disruption by shift work is associated with greater risk for cancer development and poor prognosis, suggesting a putative tumor-suppressive role for circadian rhythm homeostasis. Using a genetically engineered mouse model of lung adenocarcinoma, we have characterized the effects of circadian rhythm disruption on lung tumorigenesis. We demonstrate that both physiologic perturbation (jet lag) and genetic mutation of the central circadian clock components decreased survival and promoted lung tumor growth and progression. The core circadian genes Per2 and Bmal1 were shown to have cell-autonomous tumor-suppressive roles in transformation and lung tumor progression. Loss of the central clock components led to increased c-Myc expression, enhanced proliferation, and metabolic dysregulation. Our findings demonstrate that both systemic and somatic disruption of circadian rhythms contribute to cancer progression.
Purpose of review: The purpose of this review is to summarize recent developments linking disturbances of sleep and circadian rhythms to an increased risk for obesity, and to review novel research on potential countermeasures. Recent findings: Effective treatments for obesity are limited, with long-term adherence to lifestyle changes proving difficult to maintain. Identifying new preventive strategies based on modifiable risk factors is therefore imperative in the fight against obesity. Disturbances of sleep and circadian rhythms have an adverse impact on food choices, hunger and appetite, and have lifelong deleterious metabolic consequences when they occur during childhood and early adulthood. The upregulation of the endocannabinoid system and abnormalities in the temporal distribution of caloric intake were recently implicated in the link between sleep loss and obesity risk. In addition, alterations in circadian variation in the composition and functionality of the gut microbiome have been identified as potential contributors to metabolic dysfunction during jet lag and shift work. Insufficient sleep and circadian misalignment are thus new modifiable risk factors for obesity. Emerging evidence suggests that novel countermeasures, such as manipulations of the timing of food intake, may be effective strategies in the prevention of obesity. Summary: Four important findings are briefly reviewed: disturbances of sleep and circadian rhythms in children and young adults are risk factors for the development of lifelong obesity; circadian misalignment, as occurs in shift work, has an adverse impact on energy balance and increases the risk of weight gain; the endocannabinoid system, an important regulator of hedonic feeding, could be a potential link between sleep, circadian rhythms, and feeding behavior; and disturbances of the circadian variation in composition of the gut microbiome may be involved in the increased risk of obesity associated with insufficient sleep and circadian misalignment.
Prevalence and impact of metabolic disease is rising. In particular, overweight and obesity are at epidemic levels and are a leading health concern in the Western world. Shift work increases the risk of overweight and obesity, along with a number of additional metabolic diseases, including metabolic syndrome and type 2 diabetes (T2D). How shift work contributes to metabolic disease has not been fully elucidated. Short sleep duration is associated with metabolic disease and shift workers typically have shorter sleep durations. Short sleep durations have been shown to elicit a physiological stress response, and both physiological and psychological stress disrupt the healthy functioning of the intestinal gut microbiota. Recent findings have shown altered intestinal microbial communities and dysbiosis of the gut microbiota in circadian disrupted mice and jet lagged humans. We hypothesize that sleep and circadian disruption in humans alters the gut microbiota, contributing to an inflammatory state and metabolic disease associated with shift work. A research agenda for exploring the relationship between insufficient sleep, circadian misalignment and the gut microbiota is provided.
Objectives Oxidative DNA damage may be increased among nightshift workers because of suppression of melatonin, a cellular antioxidant, and/or inflammation related to sleep disruption. However, oxidative DNA damage has received limited attention in previous studies of nightshift work. Methods From two previous cross-sectional studies, urine samples collected during a night sleep period for 217 dayshift workers and during day and night sleep (on their first day off) periods for 223 nightshift workers were assayed for 8-hydroxydeoxyguanosine (8-OH-dG), a marker of oxidative DNA damage, using high-performance liquid chromatography with electrochemical detection. Urinary measures of 6-sulfatoxymelatonin (aMT6s), a marker of circulating melatonin levels, and actigraphy-based sleep quality data were also available. Results Nightshift workers during their day sleep periods excreted 83% (p=0.2) and 77% (p=0.03) of the 8-OH-dG that dayshift workers and they themselves, respectively, excreted during their night sleep periods. Among nightshift workers, higher aMT6s levels were associated with higher urinary 8-OH-dG levels, and an inverse U-shaped trend was observed between 8-OH-dG levels and sleep efficiency and sleep duration. Conclusions Reduced excretion of 8-OH-dG among nightshift workers during day sleep may reflect reduced functioning of DNA repair machinery, which could potentially lead to increased cellular levels of oxidative DNA damage. Melatonin disruption among nightshift workers may be responsible for the observed effect, as melatonin is known to enhance repair of oxidative DNA damage. Quality of sleep may similarly impact DNA repair. Cellular levels of DNA damage will need to be evaluated in future studies to help interpret these findings.