By Amanda MacMillan
November 01, 2016
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This coming weekend we’ll turn our clocks back to signify the end of daylight savings time. The good news? We get an extra hour of sleep. The bad news? It will suddenly be dark an hour earlier.

We already knew that losing an hour of afternoon sunshine can be a real drag, but a new study suggests just how significant that emotional effect might really be: It turns out, depression diagnoses increase significantly in the month following the autumn time change, even after weather and total amount of daylight are taken into account.

For some people, feeling blue can indeed be seasonal. Shorter days and less time spent outdoors are triggers for seasonal affective disorder, a condition that’s estimated to affect up to 5 percent of the population. And even people who don’t have clinical depression can experience milder feelings of sadness, fatigue, and low mood throughout the colder months.

But this new study suggests that the time change itself—which doesn’t affect the total amount of light in a day, but shifts the clock so that the sun both rises and sets an hour earlier—has its own impact on depression risk, above and beyond the gradual transition to winter.

While other research has linked the start and end of daylight saving time to other health effects (like sleep disturbances, risky driving behavior, and even stroke risk), this is the first to look specifically at depression in the month directly following the fall transition.

To examine this phenomenon, researchers looked at 185,419 depression diagnoses between 1995 and 2012, all made at psychiatric hospitals in Denmark, which follows a similar daylight saving time schedule as the United States.

Specifically, they looked at patterns of depression diagnoses throughout the year and in the period leading up to both seasonal time changes. And while they found no increased (or decreased) risk in depression diagnoses after the springtime “spring forward” transition, they did find one for the fall.

In the month following the “fall back” transition, the rate of depression diagnoses was, on average, 8 percent higher than expected—even after the researchers controlled for other seasonal factors. The increase in risk, which was as high as 11 percent at one point, dissipated and returned to normal over about 10 weeks.

"We are relatively certain that it is the transition from daylight saving time to standard time that causes the increase in the number of depression diagnoses and not, for example, the change in the length of the day or bad weather. In fact, we take these phenomena into account in our analyses," co-author Søren D. Østergaard, MD, associate professor of clinical medicine at Aarhus University in Denmark, said in a press release.

Even though the study looked at relatively severe depression diagnoses, Dr. Østergaard believes that the time change likely has similar effects on milder forms, as well.

"We expect that the entire spectrum of severity is affected by the transition from daylight saving time to standard time,” he says, “and since depression is a highly prevalent illness, an increase of 8 percent corresponds to many cases.”

The study was not able to show a definite cause-and-effect relationship between the time change and the increased rate of depression, nor was it able to identify exactly why the time change would affect mood. But Dr. Østergaard does speculate on some possible causes.

"We probably benefit less from the daylight in the morning between 7 and 8, because many of us are either in the shower, eating breakfast or sitting in a car or bus on the way to work or school,” he says. “When we get home and have spare time in the afternoon, it is already dark.”

In other words, even though the day doesn’t technically get a full hour shorter, it suddenly feels that way. And the change back to daylight saving time in the spring can seem very far away.

“The transition to standard time is likely to be associated with a negative psychological effect, as it very clearly marks the coming of a period of long, dark and cold days," Dr. Østergaard adds. It may be especially distressing, he wrote in the journal Epidemiology, to people who have experienced seasonal depressive episodes in the past.

Gail Saltz, M.D., clinical associate professor of psychiatry at Weill-Cornell Medical College and host of the Power of Different podcast, says these findings aren’t at all surprising, and they support what mental health professionals have believed for years.

“For many people, the seasonal change in mood does seem to be light-related, so this makes complete sense,” says Saltz, who was not involved in the study. She also says that getting more light in the morning—especially once the sun comes up an hour earlier—may help.

“Sit by a window and get some natural light first thing in the morning, or get outside and exercise during the daylight hours,” she suggests.

Some people will also benefit from using a light box, a medical device that delivers a specific wavelength of ultraviolet light. But talk to your doctor before trying this therapy, says Dr. Saltz, since it’s not recommended for everyone. (It can trigger manic episodes in people with bipolar disorder, for example.)

Dr. Østergaard told that it’s “highly likely” his study findings apply to the United States population as well, and to other countries that also observe Daylight Saving Time—about 1.6 billion people worldwide. He recommends that both health care professionals and patients, especially those who have a personal or family history of depression, should be aware of the increased risk during this time.

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