Carried out by researchers at University of Colorado Boulder and the Channing Division of Network Medicine at Brigham and Women's Hospital, this new research is the largest and most detailed observational study yet to look at the link between chronotype -- an individual's own natural preference for when they prefer to sleep and when they feel more awake and active -- and mood disorders.
The team looked at 32,470 female nurses with an average age of 55 and asked them to report on their sleep patterns by completing two questionnaires two years apart. Depression risk factors such as body weight, physical activity, chronic disease, sleep duration, or night shift work were also assessed, and the women were followed for a 4-year period in total. All of the women were free from depression at the start of the study.
The responses showed that 37 percent of the women described themselves as early chronotypes, 53 percent described themselves as intermediate chronotypes, and 10 percent described themselves as late chronotypes. The late chronotypes, known as night owls, were less likely to be married, more likely to live alone, more likely to smoke, and more likely to have erratic sleep patterns.
After taking into account these factors, the team found that early risers still had a 12-27 percent lower risk of being depressed than intermediate chronotypes. Late chronotypes also had a 6 percent higher risk of depression than intermediate types, however the researchers noted that this increase was modest and not statistically significant.
As the researchers factored in various influencing factors the results suggest that chronotype -- which is partly determined by genetics -- appears to mildly influence depression risk. Previous studies have also found that certain genes which influence our sleep-wake preference may influence depression risk.
However lead author Céline Vetter stressed that the findings do not mean night owls will definitely develop depression. "Yes, chronotype is relevant when it comes to depression but it is a small effect," she says, noting that her study found a more modest effect than previous ones have.
Night owls may also be able to change their preference, with Vetter adding, "Being an early type seems to be beneficial, and you can influence how early you are. Try to get enough sleep, exercise, spend time outdoors, dim the lights at night, and try to get as much light by day as possible.
A large-scale UK study published earlier this year also found that disrupting the body's natural rhythm, for example by working night shifts or suffering repeated jetlag, could increase the risk of mood disorders, feelings of unhappiness, severe depression, and bipolar disorder, and also lead to a decline in cognitive functions such as memory and attention span.
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