Women who are self-described “early risers” are less likely to develop depression compared to night-owls.

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The findings were reported by researchers at the University of Colorado at Boulder and Brigham and Women’s Hospital in Boston who analyzed data on 32,470 female nurses enrolled in the Nurses’ Health Study survey that they complete biannually. The average age of the female nurses was 55.

The four-year-long study looked for any association between mood disorders and chronotype — the propensity for the individual to sleep at a particular time during a 24-hour period.

Among the participants, 37 percent described themselves as early risers, 53 percent said they were intermediate types, and 10 percent described themselves as evening types, or night owls.

According to the researchers, genetics partly determine which end of the spectrum each person falls into.

At the beginning of the study, there were no recorded cases of depression. At the end of it, the researchers found 2,581 cases of depression had developed, 290 of whom were in the night owl category. Even when risk factors were taken into account, such as living alone, smoking and being single, night-owls were likelier to develop depression than morning larks.

The nurses who fell in the ‘early-riser’ category were 12 to 27 percent less likely to develop depression symptoms.

This correlation suggests there may be a link between chronotype and depression risk that is not driven by environmental or lifestyle factors.

Various studies have linked sunlight exposure to mental health. For instance, messing up the normal light and dark cycles by sleeping during the day and being awake at night, under artificial light, can disrupt the body’s metabolism. There’s also a condition called Seasonal Affective Disorder, or SAD. Also known as the winter blues, this condition is best described by lethargy and feelings of sadness and hopelessness that come when the weather forces us to spend more time indoors, with little exposure to natural light.

Sunlight also plays an important role in mood regulation. One important neurotransmitter in the brain, serotonin, is more abundant during sunny days than on cloudy ones, and this effect remains no matter how cold or hot the weather was. Higher levels of serotonin are correlated with better moods and feelings of satisfaction and calmness, while lower levels are associated with depression and anxiety.

The researchers, however, underscore that being a night-owl won’t necessarily put you at a higher risk for depression. While a relevant factor in depression, chronotype has a small effect. In the future, the team wants to investigate the contribution of light patterns and genetics on the link between chronotype and depression.

The findings appeared in the Journal of Psychiatric Research.