Daylight saving time (DST)—also “summer time” in British English and European official terminology respectively – is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less.

Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn.

Modern DST was first proposed in 1895 by George Vernon Hudson and many countries have used it since then with varying details.

Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farming, evening entertainment and other occupations tied to the sun.

An early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited or contradictory.

The effect on health are not definitive.

  • DST might provide more afternoon sunlight for outdoor exercise.
  • It alters sunlight exposure; whether this is beneficial depends on one’s location and daily schedule, as sunlight triggers vitamin D synthesis in the skin, but overexposure can lead to skin cancer.
  • Sunlight strongly influences seasonal affective disorder, so SAD sufferers may find relief at this time.
  • DST may help in depression by causing individuals to rise earlier – though there is also an argument made that it has the reverse effect.
  • Clock shifts 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. (The government of Kazakhstan cited health complications due to clock shifts as a reason for abolishing DST in 2005.)