Some people are easily in bed by 10 p.m. each night. Others struggle to fall asleep before 2 or 3 a.m. Sleep researchers refer to this as an individual's chronotype. And while we generally attribute this to preference or genetics, new research suggests there may be serious health implications involved for those late sleepers.
The research, conducted by scientists at Northwestern University and the University of Surrey, tracked 433,268 men and women in the United Kingdom over a six and a half year period of time. Analysis of the data revealed that participants who identified as "definite evening types" at the start of the study had a 10 percent increased risk of mortality from all causes when compared to "definite morning types. The findings were published in the journal Chronobiology International this month.
"What we think might be happening is, there's a problem for the night owl who's trying to live in the morning lark world," Dr. Kristen Knutson, associate professor of neurology at Northwestern's Feinberg School of Medicine and a lead author of the study, told CNN. "This mismatch between their internal clock and their external world could lead to problems for their health over the long run, especially if their schedule is irregular."
In addition to a slightly higher risk of premature death, night owls in the study were also more likely to have neurological disorders, psychological disorders, diabetes, gastrointestinal disorders and respiratory disorders, Knutson said.
But other sleep experts suggest the data shouldn't cause late sleepers to panic just yet.
"The results are provocative, but they can tell us very little about why the mortality rate is higher in night owls. The study is not experimental and does not show what benefits, if any, might occur by changing one's schedule," Dr. Donald L. Bliwise, director of the program in sleep, aging and chronobiology at Emory University School of Medicine, told the AJC.
Bliwise also suggested that knowing the participants’ actual bedtimes, instead of a simple self-definition, would help researchers understand the data better.
"One person's concept of a late bedtime or early wake-up time may not be identical to another's," Bliwise said. "About 10 percent of the study population could not even answer the question, and the proportion with the highest mortality risk (those endorsing a definite evening type) was even smaller than this."
It's also unclear whether the participants' sleep patterns changed throughout the duration of the study.
"I am not sure that there is anything that night owls should do to change their sleep patterns on this basis of these observational data," Bliwise said.
Substantial scientific evidence suggests that the times when an individual goes to sleep and wakes up are strongly influenced by genetics, he added. Environmental factors, such as a job or school, affect these decisions, but people can't simply change their genetic predisposition to fit a particular schedule.
"Speaking solely on the basis of this evidence, it would be premature to force change on what may otherwise be an innate tendency to go to bed late and sleep late," Bliwise said. "If that schedule leads to chronically and sustained short sleep durations, then that might be worthy of attention."
While the study looked at a very large sample, and attempted to control for other risk factors, it merely showed a small correlation between sleeping late and a higher mortality rate. The fact that a participant's chronotype was determined by self-reporting is one of the biggest weaknesses of the study, according to the researchers.
At the same time, Knutson believes the results are enough to suggest that night owls should focus more on their health.
"An important message here is for night owls to realize that they have these potential health problems and therefore need to be more vigilant about maintaining a healthy lifestyle," she said, adding that exercising, eating right and getting adequate sleep may be particularly important to night owls.
Read the full study in the journal of Chronobiology International.