Middle-aged and older women who naturally prefer early bedtimes and wake times show a significantly reduced risk of depression, according to the largest observational study to date on chronotype—our innate sleep-wake preference—and mood disorders. This research, drawing from extensive data, reveals that genetics-influenced chronotype impacts depression risk even after accounting for factors like light exposure and work schedules.
Prior research indicated night owls are twice as likely to experience depression, but those studies relied on single-timepoint data without fully adjusting for confounding variables, leaving causality unclear: Does depression disrupt sleep, or does a late chronotype heighten risk?
To address this, researchers analyzed data from 32,470 female nurses (mean age 55) via biennial health questionnaires. In 2009, all were depression-free. Participants self-identified as early types (37%), intermediate (53%), or evening types (10%). Follow-up over four years tracked new depression cases, alongside risk factors like body weight, activity levels, chronic conditions, sleep duration, and night shifts.
Night owls were more likely to be unmarried, live alone, smoke, and have irregular sleep. After adjustments, early types had 12-27% lower depression risk than intermediates; late types showed a 6% higher risk (not statistically significant).
“This suggests a chronotype effect on depression independent of environment and lifestyle,” noted the lead researcher. Importantly, while chronotype is a factor, night owls aren't destined for depression—lifestyle changes can help.
