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It’s that time of year again. On November 6th, most of the United States will participate in that semi-annual ritual of changing the clocks by an hour. In the fall we gain an hour of sleep time, or an hour of loafing around on a Sunday morning…how bad could it be? Our circadian clock is an elaborate system of chemical signals and hormonesreacting to all sorts of environmental inputs such as light, feeding, and even temperature. The system is quite elegant, with many interconnected parts that when working well keeps us healthy with brains and metabolism in tip top condition. We can compare the circadian system to an orchestra playing a symphony…if everyone is playing the same piece, well-timed and in tune, it sounds wonderful, but if one horn is off pitch, the whole experience can be ruined. Sleep is necessary for the brain to wash away the build-up of toxic byproducts of cell metabolism accumulated over the day. Without sleep, we very quickly lose the ability to function. The effects of acute total sleep deprivation are very obvious. In folks with bipolar disorder it can cause a manic episode and seizures in those with epilepsy. Long term, even low level sleep deprivation can contribute to a myriad of bad health effects, such as obesity, depression, and dementia. It also increases risks of heart attacks and motor vehicle accidents. While one hour difference a couple times a year seems small, evidence shows us that the delicate human circadian clock doesn’t adjust well to the abrupt difference in time. When looking at the acute affects of the one hour transition of daylight savings, there are a host of papers showing negative effects on workplace injuries, productivity, traffic accidents, and heart attacks. But what about mental health? Older papers remark on no changes in suicidal behaviors or increase in inpatient or outpatient admissions during DST changes, but large Scandinavian registries over decades give us the ability to get a bigger picture of daylight savings in spring and fall and mental health. Overall admissions could balance out if each transition (forward or backward) has different effects on major depressive disorder or mood disorders with more seasonal components. It seems that the single hour change is not disruptive enough to lead to an increase hospitalization for bipolar manic episodes in this Finnish study (whereas there are cases of mania caused by bigger time shifts due to air travel). However, less dramatic but negative behavioral effects are seen in children during the days following daylight savings switches. One hour of change in the timing of the day (that, in the fall, is often looked upon favorably as ‘that extra hour of sleep’) theoretically has it’s most debilitating consequences for those with depressive disorders. We don’t understand all the intricacies of circadian rhythm and mood problems, but we do know there are many therapies involving light, sleep deprivation, early awakening, and circadian advance to an “early to bed, early to rise” sleep schedule can effectively help treat depression. Sleeping later in the morning is associated with depression, particularly in women. It makes sense, then, that a government proscribed regimen of sleeping later could increase the risk of depression, and a recent large study seems to confirm this, with an 11% increase in hospitalizations for depression in the weeks after the daylight savings transition to standard time in Denmark. Autumn daylight savings in the high latitudes shortens the effective light in the working day, with biologic and psychological effects. The one hour time change, even adding an hour of needed sleep, can be detrimental to the brain’s delicate circadian clock. It acts as one more stressor to the myriad of stress in our modern daily schedules. Given that daylight savings time may not even save energy, it’s a wonder that we subject ourselves to the disruption twice a year.