In a worrying find for shift workers, a new study recently suggested that those who are chronically sleep-deprived by their work schedule may be at an increased risk of cardiovascular disease.

The study, published this week in the American Heart Association’s journal Hypertension, said that while insufficient sleep and circadian rhythm disturbances both have been associated with adverse cardiovascular outcomes, the cause is unclear.

Researchers analyzed 26 healthy people ages 20 to 39 who were restricted to five hours of sleep for eight days. The participants were either given fixed bedtimes to mimic sleep restriction or bedtimes that were delayed by 8 1/2 hours on four of the eight days to mimic circadian misalignment. They found that 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. It was even more pronounced at night when sleep restriction was combined with delayed bedtimes.

Participants also had reduced heart rate variability at night, an increase in 24-hour urinary norepinephrine excretion in the sleep restricted and delayed bedtimes group, and reduced vagal activity related to heart rate variability during deeper sleep phases.

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“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,” Dr. Daniela Grimaldi, lead author and a research assistant professor at Northwestern University in Chicago, said in a news release. “When our sleep-wake and feeding cycles are not in tune with the rhythms dictated by our internal clock, circadian misalignment occurs.”

According to the news release, shift workers represent 15 t to 30 percent of the working population in industrialized countries. For those whose jobs demand shift work, researchers suggested workers keep a healthy diet, exercise regularly and get more sleep.

“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,” Grimaldi said.

“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,” she said. “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.”

Researchers caution it’s unclear whether results from lab studies done on shift workers would translate into real-life conditions, but next they plan to explore whether people exposed to sleep loss with or without circadian misalignment are able to recover once they get consecutive days of sleep extension.