Middle-aged and older women who naturally go to bed early and get up early are significantly less likely to develop depression, a new study shows. The study is the largest and most detailed observational study to date to investigate the association between chronotype or sleep-wake preference and mood disorders. It shows that even after taking into account environmental factors such as light exposure and work schedules, chronotype – which is determined in part by genetics – appears to slightly influence depression risk.
Previous studies have shown that night owls are twice as likely to develop depression. But because those studies often used data at a single time point and didn't take into account many other factors that influence depression risk, it has been difficult to determine whether depression causes people to stay up late or whether a late chronotype increases the risk of depression.
To clarify the question, researchers used data from 32,470 female participants, mean age 55, in which nurses completed biennial health questionnaires.
In 2009, all study participants were free of depression. When asked about their sleep patterns, 37 percent described themselves as early types, 53 percent described themselves as between types, and 10 percent described themselves as evening types. The women were followed for four years to see who had developed depression. Depressive risk factors such as body weight, physical activity, chronic illness, sleep duration or night shift work were also assessed.
The researchers found that late chronotypes, or night owls, are less likely to be married, more likely to live alone and be smokers, and more likely to have erratic sleep patterns.
After taking these factors into account, they found that early risers still had a 12–27 percent lower risk of being depressed than intermediate types. Late types had a 6 percent higher risk than intermediate types (this modest increase was not statistically significant.)
“This tells us that there may be a chronotype effect on depression risk that is not determined by environmental and lifestyle factors,” said the researcher.
The researcher emphasizes that although the study suggests that chronotype is an independent risk factor for depression, this does not mean that night owls are doomed to be depressed.
