People who identify strongly as night owls might not live as long as those who flourish in the morning, according to a study published Wednesday in Chronobiology International.
For the study, researchers reviewed data from the United Kingdom Biobank—a population-based cohort study designed to examine risk factors for major disease—on 433,268 U.K. residents ages 38 to 73 to examine the association between an individual's natural circadian rhythm, or chronotype, and increased mortality risk.
The researchers categorized the individuals into four groups, those who identified as:
- "Definite morning types";
- "Moderate morning types";
- "Moderate evening types"; or
- "Definite evening types."
The researchers conducted a baseline assessment of the individuals' health between March 2006 and October 2010 and conducted a follow-up assessment an average of six-and-a-half years later. The researchers adjusted for several factors, such as age, existing health problems, and sleep duration.
Overall, the researchers found approximately 10,000 individuals died during the follow-up period. After the researchers controlled for factors including body max index, they found individuals who defined themselves as "definite evening types" had a 10% increased risk of all-cause mortality when compared with individuals who defined themselves as "definite morning types."
When assessing risks for specific conditions, the researchers found that, when compared to individuals who wake up early, those who slept later were:
- About twice as likely to have a psychological disorder;
- 30% more likely to have diabetes;
- 23% more likely to have a respiratory disease; and
- 22% more likely to have a gastrointestinal disease.
The researchers did not find significant differences in mortality among those who identified as moderate morning or evening types.
While previous research has found associations between a person's chronotype and risk for certain diseases, the researchers said theirs is the first to link an individual's chronotype with increased mortality risk. They cautioned, however, that the study found only a correlation, not a direct cause-and-effect relationship.
While the researchers said more work is needed to determine why night owls are more likely to die early, Kristen Knutson, the study's lead author and an associate professor of neurology at Northwestern University, offered a theory: "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." She added, "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."
Knutson said, "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."
However, Jamie Zeitzer, associate professor of psychiatry and behavioral sciences at the Stanford School of Medicine, who was not involved in the study, said the findings "actually weren't as robust as I would have hoped. ... I think they would have had stronger results if, instead of just looking at [sleeping preference], they had looked at … [whether] people [are] going to bed at their correct time."
But like Knutson he suggested society plays a role in the negative health ramifications for night owls. "It's not intrinsically chronotype that's bad; it's chronotype plus our society ... and not all societies are the same," Zeitzer said. For example, he said, "If you looked in Spain, where people are much later in terms of when they go to work, my guess is that the health consequences are probably less than in the U.K."(Khan, Los Angeles Times, 4/11; Glatter, Forbes, 4/12; Bakalar, "Well," New York Times, 4/12; Lieber, CNN, 4/12; Knutson/von Schantz, Chronobiology International, 4/11).
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