[Note the lack of citations in the criticism section.]
Daylight saving time (DST), also known as summer time, is a widely used system of adjusting the official local time forward, usually by one hour from its official standard time, for the spring, summer, and early autumn periods. The term Daylight savings time, although commonly used, is technically incorrect.
DST is most commonly used in temperate regions, due to the considerable variation in the amount of daylight versus darkness across the seasons in those regions.
Governments often tout it as an energy conservation measure, on the grounds that it allows more effective use of natural sunlight resource in summer time. Since there is less darkness in the "waking day", there is less use of electric lights. Some opponents reject this argument (see below, Criticism).
Europeans commonly refer to the system as summer time: Irish Summer Time, British Summer Time, and European Summer Time. This is reflected in the time zones' names as well, e.g., Central European Time (CET) becomes Central European Summer Time (CEST)....
Rationales for DST
One of the major reasons given for observing DST is energy conservation. Theoretically, the amount of residential electricity needed in the evening hours is dependent both on when the sun sets and when people go to bed. Because people tend to observe the same bedtime year-round, by artificially moving sunset one hour later, the amount of energy used is theoretically reduced. A 1975 United States Department of Transportation study showed that DST would theoretically reduce the country's electricity usage by 1% from March to April, if implemented during these months. These numbers have been supported in Mexico, which began implementing daylight savings time in 1996. Evaluations show a national savings of 0.7% of national electric consumption (1.3 billion KWh TWh) and reduction of peak load by 500MW.
Part of the reason that it is normally observed only in the early spring, summer, and early autumn instead of the winter months is that the amount of energy saved by experiencing sunset one hour later would be negated by the increased need for artificial morning lighting due to a later sunrise. During the summer most people would wake up after the sun rises, regardless of whether daylight saving time is in effect or not, so there is no increased need for morning lighting to offset the afternoon drop in energy usage. Another reason for not observing daylight saving time in the winter is concern about children walking to school in the dark.
Another argued benefit of DST is increased opportunities for outdoor activities, including shopping in tourist areas. Most people plan outdoor activities during sunlight hours. Other benefits cited include prevention of traffic injuries (by allowing more people to return home from work or school in daylight), and crime reduction (by reducing people's risk of being targets of crimes that are more common in dark areas).
When the U.S. went on extended DST in 1974 and 1975 in response to the 1973 energy crisis, Department of Transportation studies found that observing DST in March and April saved 10,000 barrels of oil a day, and prevented about 2,000 traffic injuries and 50 fatalities saving about U.S. $28 million in traffic costs.
Criticism of DST
DST is not universally accepted and many localities do not observe it. Opponents claim that there is not enough benefit to justify the need to adjust clocks twice every year. The disruption in sleep patterns associated with setting clocks either forward or backward correlates with a small increase in the number of fatal auto accidents, (cf. above estimate of net decrease in fatal auto accidents of 50) as well as lost productivity as sleep-disrupted workers adjust to the schedule change. It is also noted that much effort is spent reminding everyone twice a year of the change, and thousands are inconvenienced by showing up at the wrong time when they forget. Since DST exchanges morning daylight for evening daylight, late sunrises occur when DST is in effect either too far before the vernal equinox or too far after the autumnal equinox and darkness in the morning can be undesirable for early risers like schoolchildren and workers who must awaken at 6:30 a.m. or earlier.
There is also a question whether the decrease in lighting costs justifies the increase in summertime air conditioning costs. Workers arriving home to an empty house during hotter hours will need to use more energy to cool their house.
It is also speculated that one of the benefits—more afternoon sun—would also actually increase energy consumption as people get into their cars to enjoy more time for shopping and the like.
DST's twice-annual shifts in recorded time cause legal and business-operational complications, as shown in the following examples. During a North American time change, a fall night during which clocks are reset from 2 a.m. DST to 1 a.m. Standard Time, times between 1 a.m. and 2 a.m. will occur twice, causing confusion in transport schedules, payment systems, etc. DST's annual autumn shift in recorded time—which causes an hour of the same numerical name to be recorded twice—also means that people born during one of those two hours have no way to know which of standard time or DST was used to record the time of their birth, unless someone such as a parent makes a note of it; birth certificates rarely keep track of this. A British politician, Lord Balfour, noted the legal complications in British law: "Supposing some unfortunate Lady was confined with twins and the first child was born 10 minutes before 3 o'clock British Summer Time. ... the time of birth of the two children would be reversed. ... Such an alteration might conceivably affect the property and titles in that House."
Daylight saving time also causes much confusion with international business, people who commute across time zones, and computer networks that span multiple time zones. One particular problem for scheduling systems is that it makes the length of a day variable. Each year there is one 23 hour day and one 25 hour day, causing display and time tracking problems, especially when coordinating events between time zones.
Some studies do show that changing the clock increases the traffic accident rate. Following the spring shift to DST, when one hour of sleep is lost, there is a measurable increase in the number of traffic accidents that result in fatalities.
People who work nights often have an extra hassle logging how many hours they worked, since it will be either one hour more or one hour less than the simple difference in start/stop times.
DST is particularly unpopular among people working in agriculture because they must rise with the sun regardless of what the clock says, and thus the people are placed out of synchronization with the rest of the community, including school times, broadcast schedules, and the like.
Other critics suggest that DST is, at its heart, government paternalism and that people rise in the morning as a matter of choice because many people enjoy night-time hours and their jobs do not require them to make the most of daylight. Different people start their day at different times (office workers start their day later than factory workers, who start their day later than farm workers), regardless of daylight saving time.