page_title: Daylight saving time
section_title: Health
hierarchical_section_title: Daylight saving time / Dispute over benefits and drawbacks / Health
caption_attribution_description: English: The biological basis for “Spring-Forward/Fall-Back”: The “Spring-Forward/Fall-Back” adjustment is explained as a correction of rigid mechanical time toward flexible natural time so that clock time shifts toward the natural synchronization of our body clocks to the sunrise as it changes throughout the seasons. The natural cycle shown in Chart #2 is more closely matched by Chart #4 (with the Spring-Forward/Fall-Back shift) rather than the unshifted Chart #3. This shows that the biggest justification for shifting our clocks twice each year is that it is a "naturally adjusted clock time". All arguments about whether or not money is saved is secondary to this primary understanding that if we had no clocks, we would naturally wake up throughout the year in synch with the sunrise. "Daylight Savings Time" serves to be a "Naturally Adjusted Clock Time". As Chart #1 shows, there is no need for such a shift for those living near the equator, because the length of daytime at low latitudes does not vary as much as it does at higher latitudes. This world map clearly shows how the vast majority of the global population lives significantly north of the equator, and therefore realizes this benefit from the practice of shifting their clocks: World population density 1994 - with equator.png Natural time is called flexible (where "the hours bend") because hours historically were divisions of time from sunrise to sunset. Sunrise to noon was divided equally, as was noon to sunset. Also sunset to midnight and midnight to sunrise. So Summer had longer daytime hours and short nighttime hours. Winter had shorter day-hours with longer night-hours.
context_page_description: Daylight saving time, also daylight savings time or daylight time and summer time, is the practice of advancing clocks during warmer months so that darkness falls later each day according to the clock. The typical implementation of DST is to set clocks forward by one hour in the spring and set clocks back by one hour in autumn to return to standard time. In other words, there is one 23-hour day in late winter or early spring and one 25-hour day in the autumn.
  George Hudson proposed the idea of daylight saving in 1895. The German Empire and Austria-Hungary organized the first nationwide implementation starting on April 30, 1916. Many countries have used it at various times since then, particularly since the 1970s energy crisis. DST is generally not observed near the equator, where sunrise and sunset times do not vary enough to justify it. Some countries observe it only in some regions; for example, parts of Australia observe it, while other parts do not. Only a minority of the world's population uses DST; Asia and Africa generally do not observe it.
context_section_description: Experts in circadian rhythms and sleep are warning about the negative health implications of DST. DST cuts on sleep and causes an increased mismatch between the body clock and local time, a condition called social jetlag. Both sleep deprivation and social jetlag have been associated with negative effects on physical and mental health outcomes, including increased risks for diabetes, obesity, heart disease, depression, and some forms of cancer. Year-round standard time (not year-round DST) is proposed to be the preferred option for public health and safety.
  In societies with fixed work schedules it provides more afternoon sunlight for outdoor exercise. It alters sunlight exposure; the effects depend on one's location and daily schedule, as sunlight triggers vitamin D synthesis in the skin, but overexposure can lead to skin cancer. DST may help in depression by causing individuals to rise earlier, but some argue the reverse. The Retinitis Pigmentosa Foundation Fighting Blindness, chaired by blind sports magnate Gordon Gund, successfully lobbied in 1985 and 2005 for U.S. DST extensions. DST shifts are associated with higher rates of ischemic stroke in the first two days after the shift, though not in the week thereafter.
  Clock shifts were found to increase the risk of heart attack by 10 percent, and to disrupt sleep and reduce its efficiency. Effects on seasonal adaptation of the circadian rhythm can be severe and last for weeks. A 2008 study found that although male suicide rates rise in the weeks after the spring transition, the relationship weakened greatly after adjusting for season. A 2008 Swedish study found that heart attacks were significantly more common the first three weekdays after the spring transition, and significantly less common the first weekday after the autumn transition. A 2013 review found little evidence that people slept more on the night after the fall DST shift, even though it is often described as allowing people to sleep for an hour longer than normal. The same review stated that the lost hour of sleep resulting from the spring shift appears to result in sleep loss for at least a week afterward. Even so, a 2014 study conducted in the United States showed that heart attacks decreased significantly after the fall DST shift. 
  The government of Kazakhstan cited health complications due to clock shifts as a reason for abolishing DST in 2005. In March 2011, Dmitri Medvedev, president of Russia, claimed that "stress of changing clocks" was the motivation for Russia to stay in DST all year long. Officials at the time talked about an annual increase in suicides.
  An unexpected adverse effect of daylight saving time may lie in the fact that an extra part of morning rush hour traffic occurs before dawn and traffic emissions then cause higher air pollution than during daylight hours.
  In 2017, researchers at the University of Washington and the University of Virginia reported that judges who experienced sleep deprivation as a result of DST tended to issue longer sentences.
caption: Justification for daylight saving time to the effect that it is a more natural adjustment for people rising with the sun
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