The body's involuntary processes may malfunction in shift workers and other chronically sleep-deprived people, and may lead to an increased risk of cardiovascular disease, according to new research in the American Heart Association's journal Hypertension
Insufficient sleep and circadian rhythm (approximately 24-hour) disturbances both have been associated with adverse cardiovascular outcomes but the cause is unclear. To determine the impact of circadian rhythm disturbances on cardiovascular function in sleep-deprived people, researchers studied 26 healthy people, age 20-39. The study participants were restricted to five hours of sleep for eight days (sleep restriction) with either fixed bedtimes (circadian alignment) or bedtimes delayed by 8.5 hours on four of the eight days (circadian misalignment).
Researchers found sleep restriction combined with delayed bedtimes when compared to sleep restriction without delayed bedtimes was associated with:
- an increased heart rate during the day for both fixed bedtimes and delayed bedtimes groups and even more so at night when sleep resctriction was combined with delayed bedtimes;
- reduced heart rate variability at night;
- an increase in 24-hour urinary norepinephrine excretion in the sleep resticted and delayed bedtimes group; and
- reduced vagal activity related to heart rate variability during deeper sleep phases (NREM); these deeper sleep phases have a restorative effect on cardiovascular function in normal individuals.
"In humans, as in all mammals, almost all physiological and behavioral processes, in particular the sleep-wake cycle, follow a circadian rhythm that is regulated by an internal clock located in the brain," said Daniela Grimaldi, M.D., Ph.D., lead author and a research assistant professor at Northwestern University in Chicago. "When our sleep-wake and feeding cycles are not in tune with the rhythms dictated by our internal clock, circadian misalignment occurs."
Researchers said insufficient sleep is particularly common in shift workers, who represent 15 percent to 30 percent of the working population in industrialized countries.
"Our results suggest shift workers, who are chronically exposed to circadian misalignment, might not fully benefit from the restorative cardiovascular effects of nighttime sleep following a shift-work rotation," said Grimaldi who also collaborates with the Sleep Metabolism and Health Center of the University of Chicago, where the study was conducted.
"In modern society, social opportunity and work demand have caused people to become more active during late evening hours leading to a shift from the predominantly daytime lifestyle to a more nocturnal one. Exposure to consecutive days of sleep loss can impair cardiovascular function and these negative effects might be enhanced when changes in feeding and/or sleep-wake habits lead to a circadian disruption."
Since shift work often can't be avoided, researchers suggest counteracting measures such as a healthy diet, regular exercise and more sleep be encouraged among shift-workers.
Next, researchers want to see whether people exposed to sleep loss with or without circadian misalignment are able to recover once they get consecutive days of sleep extension.
It's also unclear whether the results from lab studies done on shift workers would translate into real-life conditions.
Co-authors are Jason Carter, Ph.D.; Eve Van Cauter, Ph.D. and Rachel Leproult, Ph.D. Author disclosures are on the manuscript.
National Institutes of Health grants supported this study.