The main purpose of Daylight Saving Time (called "Summer Time" many places in the world) is to make better use of daylight. We change our clocks during the summer months to move an hour of daylight from the morning to the evening. Countries have different change dates. Glide your cursor over the map to see how changing the clocks affects different latitudes.

If you live near the equator, day and night are nearly the same length (12 hours). But elsewhere on Earth, there is much more daylight in the Summer than in the Winter. The closer you live to the North or South pole, the longer the summers. Thus, Daylight Saving Time (Summer Time) is not helpful in the tropics, and countries near the equator do not usually change their clocks.

A poll done by the U.S. Department of Transportation indicated that Americans liked Daylight Saving Time because "there is more light in the evenings / can do more in the evenings." A 1976 survey of 2.7 million citizens in New South Wales found 68% liked daylight saving.

Daylight Saving Time also saves energy. Studies done by the U.S. Department of Transportation show that Daylight Saving Time trims the entire country's electricity usage by a significant, but small amount, of less than one percent each day with Daylight Saving Time. We save energy in both the evening and the morning because we use less electricity for lighting and appliances. Similarly, In New Zealand, power companies have found that power usage decreases 3.5% when daylight saving starts. In the first week, peak evening consumption commonly drops around 5%.

Energy use and the demand for electricity for lighting our homes is directly connected to when we go to bed and when we get up. Bedtime for most of us is late evening through the year. When we go to bed, we turn off the lights and TV. In the average home, 25 percent of all the electricity we use is for lighting and small appliances, such as TVs, VCRs and stereos. A good percentage of energy consumed by lighting and appliances occurs in the evening when families are home. By moving the clock ahead one hour, we can cut the amount of electricity we consume each day.

In the summer, people who rise before the sun rises are using more energy in the morning than if DST was not in effect. However, although 70% of Americans rise before 7 am, this waste of energy from having less sunlight in the morning is more than offset by the savings of energy that results from more sunlight in the evening.

In the winter, the afternoon Daylight Saving Time advantage is offset by the morning's need for more lighting. In spring and fall, the advantage is less than one hour. So, Daylight Saving Time saves energy for lighting in all seasons of the year except for the four darkest months of winter (November, December, January and February) when the afternoon advantage is offset by the need for lighting because of late sunrise.

Daylight Saving Time "makes" the sun "set" one hour later and therefore reduces the period between sunset and bedtime by one hour. This means that less electricity would be used for lighting and appliances late in the day.

We also use less electricity because we are home fewer hours during the "longer" days of spring and summer. Most people plan outdoor activities in the extra daylight hours. When we are not at home, we don't turn on the appliances and lights.

There is a small public health benefit to Daylight Saving time. Several studies in the U.S. and Britain have found that daylight, almost certainly because of improved visibility, substantially decreases (by four times) the likelihood of pedestrians being killed on the roads. Even if it is beneficial overall, Daylight Saving Time shifts this danger from the evening to the morning.

Opposition to Daylight Saving

Occasionally people complain about daylight saving time. A frequent complaint is the inconvenience of changing many clocks, and adjusting to a new sleep schedule. For most people, this is a mere nuisance, but some people with sleep disorders find this transition very difficult.

Another complaint is sometimes put forth by people who wake at dawn, or whose schedules are otherwise tied to sunrise, such as farmers. Farmers often dislike the clocks changing mid year. Canadian poultry producer Marty Notenbomer notes, "The chickens do not adapt to the changed clock until several weeks have gone by so the first week of April and the last week of October are very frustrating for us."

In Israel, ultra-Orthodox Sephardic Jews have campaigned against daylight saving time because they recite Slikhot penitential prayers in the early morning hours during the Jewish month of Elul.

A writer in 1947 wrote, "I don't really care how time is reckoned so long as there is some agreement about it, but I object to being told that I am saving daylight when my reason tells me that I am doing nothing of the kind. I even object to the implication that I am wasting something valuable if I stay in bed after the sun has risen. As an admirer of moonlight I resent the bossy insistence of those who want to reduce my time for enjoying it. At the back of the Daylight Saving scheme I detect the bony, blue-fingered hand of Puritanism, eager to push people into bed earlier, and get them up earlier, to make them healthy, wealthy and wise in spite of themselves." (Robertson Davies, The Diary of Samuel Marchbanks, 1947, XIX, Sunday.)

Sometimes people recommend a "compromise" wherein we would set out clocks 1/2 hour forward year round. While this may sound appealing at first , it is not a good solution. In the winter months, when daylight saving is not occurring, our clock is divided such that noon should be the middle of the day (although since time zones are so wide, this does not always happen). In the summer, when the daylight is so long, we want to shift a full hour to the evening.

Some countries set their clocks to fractional time zones, for example, Kathmandu, Nepal is 5:45 hours ahead of Universal Time; and Calcutta (Kolkatta), India is 5:30 ahead. This is because their country straddles international time zones; it is not an attempt to compromise and have half Daylight Saving Time year-round.

Idea of Daylight Saving Time

The idea of daylight saving was first conceived by Benjamin Franklin (portrait at right) during his sojourn as an American delegate in Paris in 1784, in an essay, "An Economical Project." Read more about Franklin's essay.

Some of Franklin's friends, inventors of the oil lamp, were so taken by the scheme that they continued corresponding with Franklin even after he returned to America.

The idea was first advocated seriously by a London builder, William Willett (1857-1915), in the pamphlet "Waste of Daylight" (1907) that proposed advancing clocks 20 minutes on each of four Sundays in April, and retarding them by the same amount on four Sundays in September. As he was taking an early morning a ride through Petts Wood, near Croydon, Willett was struck by the fact that the blinds of nearby houses were closed, even though the sun was fully risen. When questioned as to why he didn't simply get up an hour earlier, Willett replied with typical British humor, "What?" In his pamphlet "The Waste of Daylight" he wrote:

"Everyone appreciates the long, light evenings. Everyone laments their shortage as Autumn approaches; and everyone has given utterance to regret that the clear, bright light of an early morning during Spring and Summer months is so seldom seen or used".

Early British laws and lax observance

About twelve months after Willett began to advocate daylight saving (he spent a fortune lobbying), he attracted the attention of the authorities and Mr. Pearce later Sir Robert Pearce introduced a Bill in the House of Commons to make it compulsory to adjust the clocks. The bill was drafted in 1909 and introduced in Parliament several times, but it met with ridicule and opposition, especially from farming interests. Generally lampooned at the time, Willett died on March 4, 1915.

Willett had suggested a complex scheme of adding eighty minutes, in four separate movements. On May 17, 1916, an Act was passed and scheme was put in operation on the following Sunday, May 21, 1916, following the lead of Germany. There was a storm of opposition, confusion and prejudice. The Royal Meteorological Society insisted that Greenwich time would still be used to measure tides. The parks belonging to the Office of Works and the London County Council decided to close at dusk, which meant that they would be open an extra hour in the evening. Kew Gardens, on the other hand, ignored the daylight saving scheme and decided to close by the clock.

In Edinburgh, the confusion was even more marked, for the gun at the Castle was fired at 1 p.m. summer time, while the ball on the top of the Nelson monument on Calton Hill fell at 1 o'clock Greenwich time. That arrangement was carried on for the benefit of seamen who could see it from the Firth of Forth. The time fixed for changing clocks was 2 a.m. on a Sunday.

There was a fair bit of opposition from the general public and from agricultural interests who wanted daylight in the morning, but Lord Balfour came forward with a unique concern:

[on the night the clocks are set back] Supposing some unfortunate lady was confined with twins and one child was born 10 minutes before 1 o'clock. ... the time of birth of the two children would be reversed. ... Such an alteration might conceivably affect the property and titles in that House.

After the War, several Acts of Parliament were passed relating to summer time. Eventually, in 1925, it was enacted that summer time should begin on the day following the third Saturday in April (or one week earlier if that day was Easter Day). The date for closing of summer time was fixed for the day after the first Saturday in October.

The energy saving benefits of this were recognized during World War II, when clocks were put two hours ahead of GMT during the Summer. This became known as Double Summer Time. During the war, clocks remained one hour ahead of GMT throughout the winter.