We all know (or should know by now) that to be healthy, most of us need seven to nine hours of sleep each night. Getting less than that amount over a prolonged period of time has been linked to serious medical conditions, including high blood pressure, stroke and coronary heart disease.
But how long we slumber is not the only determinant of a healthy night’s sleep. Research has also linked large swings in when we fall asleep with poorer cardiovascular health, even if the total hours we sleep remain the same.
Why fluctuating bedtimes affect our cardiovascular health is not yet clear, but a new study has identified one possible factor: a higher resting heart rate. The study, published recently in the journal npj Digital Medicine, found that people who go to bed even 30 minutes later than their usual bedtime tend to have a higher resting heart rate throughout the night and often into the following day.
That’s potentially problematic. An elevated resting heart rate is a known risk factor for cardiovascular disease. A 2019 study found, for example, that men at the age of 50 with a resting heart rate of more than 75 beats per minute were twice as likely to die of cardiovascular disease — or any other cause — during the next two decades than those with a resting heart rate of 55 beats per minute or less.
How the study was done
The authors of the current study — a team led by Nitesh Chawla, a professor of computer science and engineering at the University of Notre Dame — wanted to see what happens to people’s resting heart rate when they don’t adhere to their normal bedtimes. To do that, they analyzed data collected from Fitbit devices worn by 557 first-year college students over four years.
The devices kept track of more than 255,000 sleep sessions, measuring the students’ bedtimes, sleep duration and resting heart rate.
At the start of the study, about half (49 percent) of the students slept for an average of 7-8 hours per night, while more than a third (37 percent) slept for 6-7 hours. About 10 percent slept for 8-9 hours and 3 percent slept for less than 6 hours.
The most common bedtime was 1 a.m.-2 a.m. (42 percent of the students), followed by midnight-1 a.m. (29 percent), 2 a.m.-3 a.m. (17 percent) and 11 p.m-midnight (5 percent).
Over the course of the study’s four years, the number of students who slept at least seven hours per night grew somewhat larger, although the bedtimes didn’t change much.
The study found a correlation between the regularity of the students’ bedtime and their resting heart rate. When students went to bed 30 minutes or later after their normal bedtime, they experienced significant increases in their resting heart rate, both while they slept and into the following morning. (Normal bedtime was defined in the study as one hour before or after a person’s average bedtime.)
The later the bedtime, the higher the increase in resting heart rate.
“Through our study, we found that even if you get seven hours of sleep a night, if you’re not going to bed at the same time each night, not only does your resting heart rate increase while you sleep, it carries over into the next day,” says Chawla, in a released statement.
Interestingly, going to bed earlier than normal was also associated with an elevated resting heart rate, although the bedtime had to be more than 30 minutes earlier than normal to change the rate significantly. The effect didn’t carry into the next day, however.
The study is observational, which means it doesn’t prove that altered bedtimes were behind the changes in the participants’ resting heart rates. Also, the study involved only young college students. The results might be different in other, particularly older, groups of people.
In addition, Fitbits are not always accurate in recording sleep patterns. In fact, when compared with polysomnography, the gold-standard method of tracking sleep patterns, Fitbits have been found to overestimate total sleep time by more than 10 percent.
It’s also not clear whether the higher resting heart rates observed in the study have any clinical meaning in terms of their impact on health.
Still, Chawla and his colleagues believe their findings “stress the importance of maintaining proper sleep habits, beyond sleep duration.”
Consistency — regular bedtimes — is an important sleep habit too, they say.
“For some, it may be a matter of maintaining their regular ‘work week’ bedtime through the weekend,” notes Chawla. Yet he also acknowledges that for others — particularly shift workers and frequent travelers — getting to bed at the same time each night is a challenge.
“Establishing a healthy bedtime routine — as best you can — is obviously step number one,” says Chawla. “But sticking to it is just as important.”
FMI: You can read the study on the npj Digital Medicine website.