From MSNBC:
It’s that time of year, when crocuses bloom, the lawn starts to need mowing, and most Americans lose an hour’s sleep setting their clocks ahead. (Remember? Spring forward, fall back.) So here are answers to your questions about the time switch — and about sleep. Most Americans move their clocks ahead for daylight-saving time in the wee hours of the second Sunday in March. The day of the big switch used to be the first Sunday of April, but in 2005,
Congress revised the rule as an energy-saving measure. What's the rationale behind the switchover? 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. Not everybody goes along with the plan, as folks in places like Hawaii and most of Arizona know quite well. Each state or country comes up with its own schedule for the switch: Most European countries don't switch to summer time until the last weekend in March. And yes, some countries in the Southern Hemisphere are moving their clocks back an hour at this time of year.
More here.