An extra hour of sleep sounds great, but its effects on the mind and body are mixed.

The clocks roll back this Sunday, reversing daylight saving time (DST) and giving more than 1.5 billion people worldwide an extra hour of sleep.

For the 43% of Americans who say they rarely or never get a good night’s sleep on weeknights, the change is welcome. The extra hour of sleep can help offset chronic sleep deprivation, which has been linked to such disasters as the sinking of the Titanic and the Exxon Valdez oil spill.

But sleep-shifting carries its hazards, too. The human body’s circadian clock evolved to shift over the course of an entire season, responding to the gradual loss of daylight by slowly shifting our sleep and wake times to match. Losing an hour of sunlight and gaining an hour of sleep in a single night is a far faster shift than our internal clock can keep up with, throwing it off-kilter.

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Fatigue can lead to microsleeps, where the brain drops into a state of sleep for just a few seconds, usually without the person ever knowing that it happened. Historically, microsleeps weren’t a problem, but humans didn’t evolve to operate high-speed machinery like cars, where one or two seconds can be the difference between a close call and a tragic accident.

One study in Canada found that the loss of an hour of sleep in the spring caused an eight percent increase in the number of traffic accidents. In the fall, the rate dropped by the same amount, as everyone caught up on the sleep they’d been missing during the long summer months.

But despite the lost sleep, DST does offer one benefit: an extra hour of sunlight during the evening. Another study found that by commuting home in daylight instead of in darkness, drivers were better able to avoid lethal car crashes, particularly those involving pedestrians.

The authors estimated that 901 fatal crashes over a five-year period, of which 727 killed pedestrians, could have been avoided if daylight saving time had been in effect year-round.

If you find yourself getting sleepy behind the wheel, don’t try to push through it—instead, pull over and take a nap. Even a brief 10 to 20 minute rest can be enough to reduce your chances of microsleeping long enough to get home safely.

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In the spring, the loss of an hour of sleep and the disruption of the body’s circadian clock cause a big spike in the rate of heart attacks.

“For some individuals, it might be more than just an hour of lost sleep,” said Professor Imre Janszky of the Karolinska Institute in Sweden in an interview with Healthline. “Several studies indicated that the quality of sleep is affected in some, not only the length.”

As with traffic accidents, after DST ends, the rate of heart attacks drops again. The effect is more drastic in people younger than 65, but anyone at risk for heart disease should take precautions.

“A gradual change might help,” said Janszky. “It might be wise to start the adjustment gradually before and perhaps—if one can afford—to continue after the official shift.”

In the brain, the systems that control mood and sleep are directly tied to the body’s clock, to the extent that some people experience Seasonal Affective Disorder (SAD), or winter depression. For people who are vulnerable to SAD, losing an hour of sunlight all at once instead of gradually can be the event that triggers their depression.

The effects of depression reach beyond just a bad mood. One study found that rates of male suicide increase after DST goes into effect in the spring. Surprisingly, the fall time shift reveals the same trend: despite gaining an hour of sleep, the disruption to the body’s clock boosts suicide rates again.

Professor Michael Berk of the University of Melbourne in Australia told Healthline that any disruption in sleep habits can increase these risks, and the single-hour shift of DST is far from the biggest culprit. “Sleep matters, and routine matters. The internet, wild weekends, and TV cause much bigger disruptions to sleep,” he said.

Janszky agrees. “Sudden circadian upset and acute sleep deprivation, which can be much more pronounced at other occasions than at the DST shifts, might carry some risk of heart attacks, especially for those who are at risk anyway.”

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