People who don’t keep regular times for going to bed and waking up could be putting themselves at risk of metabolic disorders including obesity, hypertension, high blood sugar, and high cholesterol, according to research results released today. Multiple previous studies have linked shorter sleep duration with an increased risk of obesity and diabetes, but the new study, involving more than 2,000 people, suggests that every hour of variability in time to bed and time asleep may increase the risk of a metabolic abnormality by 27%.

“Our research shows that, even after considering the amount of sleep a person gets and other lifestyle factors, every one-hour night-to-night difference in the time to bed or the duration of a night’s sleep multiplies the adverse metabolic effect,” commented Tianyi Huang, ScD, an epidemiologist at the Channing Division of Network Medicine at Brigham and Women’s Hospital.

Huang and co-researcher Susan Redline, MD, senior physician in the Division of Sleep and Circadian Disorders at Brigham and Women’s Hospital, report on their findings in Diabetes Care, in a paper titled, “Cross-sectional and prospective associations of actigraphy-assessed sleep regularity with metabolic abnormalities: The Multi-Ethnic Study of Atherosclerosis.”

Adequate sleep is important for normal metabolic functioning and maintaining energy balance. Research has shown that too little sleep can have adverse effects on metabolic health. However, less is known about whether high day-to-day variations in sleep timing and duration might also affect metabolic health, for example, by disrupting the body’s circadian rhythms. “It is now well established that an inherent circadian rhythmicity is a universal mechanism underlying various biologic processes, including metabolism,” the authors wrote. “… irregular sleep schedules may be the most common cause of disruption of the circadian system in the general population, potentially leading to chronic, cumulative metabolic effects.”

person yawning
“Biological night” is defined as the period between the onset and cessation of melatonin secretion. During this period, melatonin is secreted, blood cortisol levels rise, core body temperature goes down, and we become sleepy. Melatonin is produced only during darkness and stops upon optic exposure to bright light, with light in the blue portion of the visible spectrum proving the most potent at suppressing production. [Angela Spivey, 10.1289/ehp.118-a28]
There is already some evidence to suggest that, independently of sleep duration, irregular sleep timing and duration might be linked with obesity, hypertension, and dyslipidemia, but relevant studies haven’t been designed to demonstrate causality, Huang and Redline pointed out. For their reported study, the investigators followed 2,003 men and women, aged 45–84, who participated in the National Heart, Lung, and Blood Institute (NHLBI)-funded Multi-Ethnic Study of Atherosclerosis (MESA). The participants were studied for a median of 6.3 years to investigate whether there was any association between sleep regularity and metabolic abnormalities, independently to sleep duration.

Each individual underwent several tests. They wore an actigraph wristwatch to monitor sleep schedules seven consecutive days, and also underwent polysomnography—a type of test used to diagnose sleep-related breathing disorders—for one night at home. The participants also kept a sleep diary and answered a standard questionnaire about their sleep habits and other lifestyle factors. Actigraphy tracking was carried out between 2010 and 2013, and the participants were then followed until 2016 and 2017.

The researchers had hypothesized that participants with higher variations in their nightly sleep duration and time that they went to sleep would have a higher incidence of metabolic syndrome, even after controlling for the amount of sleep they got. The prospective results confirmed the hypothesis, showing that variations in bedtimes and sleep duration preceded the development of metabolic dysfunction.

After adjustment for common lifestyle and sleep-related factors that might impact on metabolic health and sleep regularity, the results still indicated that increasing sleep duration or bedtime variability was strongly associated with changes to multiple metabolic indicators, and a greater incidence of lower HDL cholesterol, higher waist circumference and increased blood pressure, total triglycerides and fasting glucose. “ … sleep duration variability >120 minutes and sleep timing variation >90 minutes were consistently associated with metabolic syndrome …” the authors wrote.

Participants whose sleep duration varied more than one hour were more likely to be African-Americans, work non-day shift schedules, smoke, and have shorter sleep duration. They also exhibited higher depressive symptoms, total caloric intake, and index of sleep apnea.

The researchers say their prospective study results support findings from prior cross-sectional studies, and suggest that sleep irregularity does have a detrimental effect on metabolic regulation. The latest findings also point to a causal link, they stated “… the prospective results identified that variation in sleep preceded the development of metabolic dysfunction, providing temporal evidence supporting a causal link between irregular sleep and metabolic dysfunction.”

Huang and Redline acknowledge that their study does have some limitations. Nevertheless, noted Michael Twery, PhD, director of the NHLBI’s National Center on Sleep Disorders Research, “Objective metrics and a big and diverse sample size are strengths of this study … As is the study’s ability to look not only at current factors, but to conduct a prospective analysis that allowed us to assess whether patterns of irregular sleep could be linked to future metabolic abnormalities.”

Interestingly, high variability in sleep patterns tended to be associated with multiple metabolic abnormalities, rather than just one or two. “Compared with the cluster of few metabolic changes, every one-hour increase in sleep variability was associated with almost doubled odds for the cluster characterized by incidence of multiple metabolic abnormalities,” they wrote. “ … this finding suggests that sleep irregularity may influence physiologic pathways that result in clusters of metabolic abnormalities .” This is particularly significant given that more than half of the individuals involved showed an average night-to-night variability in sleep duration of more than an hour. And with all of the participants being over 45 years of age, it is possible that younger populations may have even higher sleep variability when they are studying or working, the authors pointed out.

The findings thus have important public health implications, and could be used to help draft simple, quantitative guidelines for healthy sleep, the investigators concluded. “Although the majority of the population do not regularly experience such extreme circadian misalignment as rotating night shift work or frequent jet lag, irregular sleep is a highly prevalent form of chronic circadian disruption in today’s society … our results suggest that maintaining a regular sleep schedule has beneficial metabolic effects, which may enrich current prevention strategies for metabolic disease that primarily focus on promoting sufficient sleep and other healthy lifestyles … Given that sleep regularity represents a modifiable risk factor, future studies should evaluate effective strategies to reduce sleep variability, taking into account social and behavioral factors that may hinder sleep regularity.”

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