What changing the clocks back does to your health

Millions of Americans will welcome an extra hour of sleep this weekend as they turn back their clocks for the end of Daylight Saving Time. But the time change may take a toll on health in a number of ways.

A study out of Britain finds that with the time change and the days getting shorter, children get less exercise.

Researchers at the London School of Hygiene and Tropical Medicine studied 23,000 children, ages five to 16, from 9 countries including the U.S. The kids wore an electronic monitoring device to track their body movement. The researchers determined that every lost hour of daylight corresponded to a 5 percent decline in kids' activity level.

That adds up to about 2 minutes of running around each day. It may not sound like a lot, but researchers say children only get about a half an hour of vigorous activity a day, so even a little bit more would help.

"This clearly is not big enough to be the whole solution to the problem of a lack of physical activity and obesity but it's absolutely a step in the right direction," Dr. Anna Goodman told CBS News' Lucy McDonald.

Geoff Glaster, a father of two, didn't need a scientific study to know what it does to his kids. He says he dreads the fall when the days get shorter and his boys are cooped up indoors.

"The more kids are outside enjoying what little daylight they have, the better off they're going to be," he said.

Earlier nightfall and shorter days also cause problems for people with Seasonal Affective Disorder, or SAD, a form of depression that sets in for about half a million Americans every fall and winter.

SAD is triggered by a lack of sunlight, although why some people are affected this way is still a mystery. One theory is that without adequate sunlight, a person's internal biological clock loses its bearings and its ability to regulate mood and sleep. Another possible explanation is that sunlight stimulates chemicals in the brain such as serotonin, a neurotransmitter believed to help maintain mood balance.

The standard treatment for SAD is light therapy -- soaking up artificial sunlight from fluorescent light boxes for anywhere from 15 minutes to an hour or more each day. Most patients see symptoms disappear once the days start getting longer again.

So will switching back to Daylight Saving Time in the spring make everyone a little bit healthier? It doesn't quite work that way.

According to a study out last March, losing an hour of sleep when the clocks spring forward may increase the short-term risk of suffering a heart attack. The study analyzed data from hospitals in Michigan, and found a 25 percent uptick in heart attacks the Monday after Daylight Saving Time began.

Monday mornings are already the most common time for heart attacks, perhaps because of "a combination of factors, including the stress of starting a new work week and inherent changes in our sleep-wake cycle," lead author Dr. Amneet Sandhu, a cardiology fellow at the University of Colorado in Denver, said in a news release. "With Daylight Saving Time, all of this is compounded by one less hour of sleep."

He said hospitals should consider increasing their staffing the Monday after the time change in order to better care for the surge in heart patients.