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Spring into daylight science time

Brian Snyder / Reuters

Peter Shugrue checks four custom-made clocks, destined for installation in Kansas City, Mo, at the Electric Time Company factory in Medfield, Mass. Daylight saving time begins in the United States at 2 a.m. Sunday.

It's that time of year, when most Americans lose an hour's sleep setting their clocks ahead. (Remember? Spring forward, fall back.) So here are answers to questions about the time switch and about sleep.

In most of the United States, we'll be moving our clocks ahead for daylight saving time in the wee hours of Sunday morning. The official switch comes at 2 a.m. Sunday, which instantly becomes 3 a.m. Most people, however, move their clocks ahead just before going to bed Saturday night or just after getting up Sunday morning.


The day of the big switch used to be the first Sunday of April, but in 2005, Congress changed the rule to make it the second Sunday in March, as an energy-saving measure.

What's the rationale?
As the year progresses toward the June solstice, the Northern Hemisphere gets longer periods of sunlight. Timekeepers came up with daylight saving time — or summer time, as it’s known in other parts of the world — to shift some of that extra sun time from the early morning (when timekeepers need their shut-eye) to the evening (when they play softball).

The idea is that having the extra evening sunlight will cut down on the demand for lighting, and hence cut down on electricity consumption — and that few people will miss having it a little darker at, say, 6 o'clock in the morning. At least that's how the theory goes.

Who's in on the switch?
Not everybody goes along with the plan. Arizona sticks with Mountain Standard Time, which turns out to be the same as Pacific Daylight Time. (The Navajo Nation, however, goes along with the summertime switch.) Hawaii and U.S. possessions such as American Samoa, Guam, Puerto Rico and the Virgin Islands are also staying on standard time.

Most European countries don't switch to summer time until the last weekend in March. That means the usual time difference will be out of sync for three weeks. For example, when it's noon in New York, it'll be 4 p.m. in London. But starting March 31, the five-hour difference between the two cities' clocks will be back in force.

Some countries in the Southern Hemisphere move their clocks back an hour at this time of year. In Brazil, for example, the switch from daylight saving time to standard time took place in mid-February.

How can we cope?
If you’re in the “spring forward” mode, don’t lose any sleep over the hour you’re losing. But do try to get back into your regular sleep routine. Rosalind Cartwright, a sleep expert at Rush Medical Center in Chicago, says that if you lose too much sleep, even a couple of hours for just two or three days, your immune system will suffer and you'll be more susceptible to colds and viral infections.

A couple of small-scale studies have suggested that heart attack rates go up during the switch to daylight saving time, perhaps because of the sleep-cycle disruption. But the evidence is too meager to make a solid connection to that issue or other purported health effects of the time change.

Generally speaking, if you get to sleep too late, or get up too early, your body will find a way to get the deep sleep it needs for rest. But Cartwright says you lose the stage of sleep during which you dream, which is important for mood. Which explains why you might feel groggy and grumpy after we "spring forward" to daylight saving time.

More about the time changeover:


David Ropeik is a consultant and author specializing in risk perception and risk communication. Alan Boyle is NBCNews.com's science editor. This is an updated version of a report originally published in March 2000.