Failing to stick to a regular time for going to bed and waking up increases the risk of stroke, heart attack and heart failure by 26%, even for those who get a full night’s sleep, the most comprehensive study of its kind suggests.
Previous studies have focused on the links between sleep duration and health outcomes, with people advised to get between seven and nine hours shut-eye a night.
That advice still stands. But researchers are increasingly focusing on sleep patterns, and in particular the impact of irregular sleep – defined as variations in the time a person goes to sleep and wakes up.
The new study found irregular sleep – going to bed and waking up at different times each day – was “strongly associated” with a higher risk of major adverse cardiovascular events. Even getting eight hours of sleep was insufficient to offset the harmful effects of consistently varying bed and wake-up times, experts said.
The research, published in the Journal of Epidemiology and Community Health, involved 72,269 people aged 40 to 79 from the UK Biobank study. It did not establish precisely how close you have to get to the same bed and wake-up time – only that the further away you are, the higher the risk of harm.
The lead author, Jean-Philippe Chaput, of the University of Ottawa, said: “We should aim to wake up and go to sleep within 30 minutes of the same time each night and each morning, including weekends. Within an hour of the same time is good but less good than 30 minutes, and even better is to have zero variation.
“Beyond an hour’s difference each night and each morning means irregular sleep. That can have negative health impacts. The closer you are to zero variation the better.
“No one is perfect across a whole year, and if you don’t have a regular sleep pattern for one or two days a week, it’s not going to kill you. But if you repeatedly have irregular sleep, five or six days a week, then it becomes chronic, and that is a problem.”
Chaput said waking up at the same time each day was more important than going to bed at the same time. “Waking up at different times each morning really messes with your internal clock, and that can have adverse health consequences,” he said.
“If you need to catch up on sleep you’ve missed during the week at weekends, then going to bed earlier is better than lying in – you should still be trying to wake up at the same time, even on Saturdays and Sundays.”
In the study, participants wore an activity tracker for seven days to record their sleep, with experts then calculating a sleep regularity index (SRI) score for each person.
The score captured the day-to-day variability in bedtime, wake-up time, sleep duration and wake-ups during the night, with people given a score ranging from 0 (very irregular) to 100 (perfectly regular sleep-wake pattern).
Participants were put into either an irregular sleep group (SRI score less than 71.6), moderately irregular sleep group (SRI between 71.6 and 87.3), or regular sleep group (SRI score more than 87.3). People were then followed up for eight years.
Even after taking into account factors that could influence the results, irregular sleepers were 26% more likely to suffer a stroke, heart failure or heart attack than those with regular sleep, the study found. Moderately irregular sleepers were 8% more likely to do so.
Researchers found the SRI score was a continuous measure, with people’s risk of heart attack and stroke increasing the more irregular their sleeping patterns were.
The recommended amount of sleep for 18- to 64-year-olds is seven to nine hours a night, and seven to eight hours for those aged 65 and over.
The study found a greater proportion of regular sleepers (61%) met the recommended sleep quota than irregular sleepers (48%). But this made no difference to heart health for irregular sleepers, who had the same higher risk of stroke and heart attack even if they were getting enough sleep.
In contrast, moderately irregular sleepers saw their risks drop if they got adequate sleep.
This was an observational study, and as such, could not establish cause and effect, and the researchers acknowledged various limitations to their findings.
But they concluded that the findings suggest that irregular sleep was strongly associated with a risk of major adverse cardiovascular events in adults, irrespective of whether or not recommended sleep quotas were met.
“More importantly, our results suggest that sleep regularity may be more relevant than sufficient sleep duration in modulating major adverse cardiovascular event risk.”
