Health Effects of Daylight Saving Time

cock by bed
Clock shifts, accidents happen. Tetra Images/Elena Elisseeva/Getty Images

“Spring forward, fall back” is the simple shorthand of Daylight Saving Time (DST), a policy in place in many countries aimed at conserving energy and making better use of daylight. But what is the more complex consequence of imposing the shift on our internal body clock, on our health and longevity?

History of DST

Launched during World War I to save energy for manufacturing, Daylight Saving Time involved moving the clock one hour ahead of Standard Time in March, to take advantage of daylight in the early evening. In the fall, clocks were switched back an hour (reverting to Standard Time), to have more daylight during the morning in winter months. While the adherence to Daylight Saving Time was spotty for many years, a number of nations now implement the seasonal shift. In Europe, the plan is called European Summer Time. Prompted by hopes of further energy savings, in 2005 the United States mandated an extension of Daylight Saving Time by four weeks, delaying the backward clock shift to November.

Proponents of shifting the clocks permanently forward — to extend daylight into the evening — argue that it promotes better health in children and adults, by enabling more leisure and physical pursuits outdoors.

Sleep Deprivation and Mental Health

While one hour’s loss — or gain — of sleep might seem minor, there is evidence that shifting our clock the equivalent of one time zone can have various effects on our state of mind, particularly in people vulnerable to depression. For example, an Australian analysis of data from 1971-2001 found an increase in male suicides after the spring shift to Daylight Saving Time, compared to the rest of the year. The 2008 study, published in Sleep and Biological Rhythms, suggests the impact could be due to sleep deprivation and disruptions in the subjects’ sleep/wake cycle, or circadian rhythm. The researchers also cite past data on multiple sets of twins — in which one twin had bipolar disorder — showing greater vulnerability to seasonal changes in mood in the affected twins.​

Traffic Accidents After the Time Changes in the Spring

A number of studies have suggested that traffic accidents and collisions rise immediately after Daylight Saving Time begins in March, attributing the increase to sleepy drivers suffering from the loss of an hour’s shut-eye. However, not all of the research findings are consistent. For example, a 2007 review published in The B.E. Journal of Economic Analysis & Policy examined the short-term and long-term effects of traffic accidents, of switching to Daylight Saving Time in the spring. The researchers, from the RAND Corporation, analyzed US crash data over a 28-year period, from 1976-2003. The findings? Moving the clock ahead made no significant difference to the number of automobile accidents in the short-term. A long-term small reduction was found, however, in both crashes involving pedestrians (down 8-11%), and those involving other vehicles (6-10%).

What Happens When the Clock Turns Back?

Reverting to Standard Time in the fall offers people an extra hour of sleep, but according to an analysis by two professors from Carnegie Mellon University in Pittsburgh, PA, the shift can be hazardous — at least, for pedestrians. Professors Paul Fischbeck and David Gerard have compiled an extensive database of traffic statistics and have presented their data to a number of US federal agencies. They compared to traffic accidents across the US in the months of October, with those in November. While no jump in collisions was found for vehicles, a serious increase — almost triple the risk — was seen in pedestrian fatalities between 5 pm and 6 pm, in the weeks following the fall time change. In the period between 1999 and 2005, an average of 37 more pedestrian deaths occurred around 6 pm in November, compared to the previous month.

Fischbeck attributes the rise to a lack of sunlight. “People are simply not used to driving in darkness,” he tells me. “The spike is worst for the two weeks following the time change, then drops back in December to normal levels.”

In the spring, Fischbeck says, the opposite is true: there are more traffic accidents during the morning rush hour after Daylight Saving Time is launched because early drivers are once again in the dark. His data suggest the increase in pedestrian fatalities in the spring is less than the rise in deaths that occurs during the evening rush hour after November's time change.

What should you make of these statistics? It appears that our bodies take longer to adjust to a seasonal time change, than our wristwatches. Take care to get enough sleep at these times of the year, and look both ways, before crossing the road in rush hour.​


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Article Sources
  • M Lambe. (2000) The shift to and from daylight savings time and motor vehicle crashes. Accident Analysis & Prevention 32:4, 609-611.
  • Mayer Hillman. “More daylight, better health: why we shouldn’t be putting the clocks back this weekend.” BMJ 2010; 34.
  • Michael Berk, Seetal Dodd, Karen Hallam, Lesley Berk, John Gleeson, Margaret Henry. “Small shifts in diurnal rhythms are associated with an increase in suicide: The effect of daylight saving.” Sleep and Biological Rhythms 2008; 6: 22–25.
  • Paul Fischbeck. Professor of Social and Decision Sciences/Engineering and Public Policy. Carnegie Mellon University. Personal communication November 5, 2012.
  • Sood, Neeraj and Ghosh, Arkadipta. “The Short and Long Run Effects of Daylight Saving Time on Fatal Automobile Crashes. The B.E. Journal of Economic Analysis & Policy. ISSN 1935-1682, 02/2007, Volume 7, Issue 1, p. 11.