Daylight time was first enacted in Germany in 1915, then quickly adopted by Britain and much of Europe and Canada.
Daylight time for 2011 ends at 2 a.m. on Sunday, Nov. 6. Be sure to set your clocks back one hour.
Because the sun shone for a time while most people were still asleep in the morning, it was reasoned that light could be better used during the day. The solution was to push the clocks ahead one hour in springtime, forcing people to wake an hour earlier. They would therefore expend less energy trying to light their homes, for instance, if time were adjusted to suit their daily patterns.
When the days started getting shorter in the fall and people awoke to increasing darkness, the clocks were turned back an hour to get more light in the morning.
Although first instituted in 1915, the idea of daylight time had been batted around for a more than a century. Benjamin Franklin suggested the idea more than once in the 1770s while he was an emissary to France. But it wasn't until more than a century later that the idea of daylight time was taken seriously.
William Willett, an English builder, revived the idea in 1907, and eight years later Germany was the first nation to adopt daylight time. The reason: energy conservation. Britain quickly followed suit and instituted British Summer Time in 1916.
Several areas, including parts of Europe, Canada and the United States, followed suit during the First World War. In most cases, daylight time ended with the armistice.
During the Second World War, a different form of daylight time was reinstated by Britain and clocks were set two hours ahead of Greenwich Mean Time during the summer. It was known as Double Summer Time. The time shift didn't end with the summer, as clocks were rolled back to be one hour ahead of GMT through the winter.
The Uniform Time Act, enacted by the U.S. Congress in 1966, established a system of uniform (within each time zone) daylight time throughout most of the U.S. and its possessions, exempting only those states in which the legislatures voted to keep the entire state on standard time.
Schedule changed to save energy
In Canada, it's up to each province to decide whether to use daylight time, and not all do. Most — but not all — jurisdictions in Canada and the U.S. have been moving their clocks ahead by one hour on the second Sunday in March and back by one hour on the first Sunday in November.
Legislation in the United States in 2007 moved the start of daylight time three weeks earlier in the spring and the return to standard time a week later in the fall. The change was aimed at trying to help save energy, since people aren't expected to need their lights on as early in the evening. But there is still some debate about whether the change reduces energy consumption.
A 2006 report from the U.S. Department of Energy anticipated electricity savings of four-tenths of a per cent per day of extended daylight time, totalling 0.03 per cent of annual electricity consumption.
As to the environmental impact, the non-profit group American Council for an Energy-Efficient Economy estimated the prolonged period of daylight time would cut carbon emissions by 10.8 million tonnes.
Canada followed suit, saying it was essential to co-ordinate with the U.S. and that not doing so would create too many headaches for trade and travel.
"We're not anxious to have a disconnect between us and our chief trading partner," Ontario Premier Dalton McGuinty said.
Most of Saskatchewan has not observed daylight time since 1966 and stays on central standard time all year round. Some border towns follow the time schemes of their neighbours in Manitoba or Alberta.
Saskatchewan Premier Brad Wall had promised a referendum on daylight saving time in the 2007 election. But in March 2011 he said one wasn't needed.
In Canada, areas of Quebec east of 63 degrees west longitude do not change to daylight time and remain on Atlantic standard time year round. Pockets of Ontario and British Columbia do not use daylight time.
Daylight time is observed in most of the United States. Just two states — Arizona and Hawaii — and three territories — American Samoa, Puerto Rico and the U.S. Virgin Islands — do not participate.
Some parts of Australia have adopted daylight time. Of course, it's done a little differently than in the Northern Hemisphere, where seasons are opposite. So, when daylight time starts in Canada, it comes to an end in Australia and vice versa.
When Canadians are waxing their skis in December, Australians are waxing their surfboards because it's summer there.
In Britain, the government may consider long-touted proposals for the U.K. to switch to Central European Time, a move advocates insist would bring lighter evenings and possibly offer the country's sluggish economy a boost.
Supporters say the switch, which would see British clocks synchronized with those in continental Europe, would extend the tourism season, cut road deaths and help promote outdoor activities. But opponents insist that northern regions would be badly affected. Some critics claim that the sunrise in Scotland could come as late as 10 a.m. during some winter months.
Health benefits, pitfalls
The end of daylight time may signal that Canada's long, cold winter is just around the corner. But the omens aren't all bad. You do pick up that extra hour of sleep that you lost when the clocks moved ahead in March.
Swedish researchers say there may be some health benefits to turning your clock back. They studied 20 years of records and found that the number of heart attacks dipped on the Monday after clocks moved back an hour.
Moving clocks ahead in the spring had the opposite effect. There were more heart attacks in the week after springing forward — especially during the first three days of the week.
In 2007, Dr. Till Roenneberg of Ludwig-Maximilians University in Munich, Germany, tracked the sleeping patterns of Europeans to explore the effects of moving from daylight time to standard time. The study found that while both late and early risers adjusted to the time switch in the fall, night owls had a particularly difficult time adjusting to the time shift in the spring. Researchers said the night owls are likely to feel sleep-deprived for weeks.
For pedestrians, though, the story's a little different. A study by U.S. researchers found there were more pedestrian deaths during the evening rush hour in November than October as drivers and pedestrians adjusted to the earlier darkness. They said the risk for pedestrians drops in the spring when clocks are set back and daylight comes earlier.