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Daylight Saving “Appears to Cause Brain Damage”


Daylight saving time (DST) will end on Sunday in most of North America, and just ended in much of Europe. This is the “good” part of daylight saving, when clocks go back and we gain an hour, but it’s still a sudden shift in our daily rhythms. Do the disruptions of daylight saving affect how well our brains work?

We’ve discussed before how a change in schedule can result in “social jet lag,” affecting sleep, which affects cognition. DST provides a natural experiment, in which large parts of the country change their sleep schedules all at once.

At this time of year, the Northern Hemisphere gets an extra hour to work with. That can be good; for example, one study found slightly lower incidence of heart attacks the Monday after daylight saving ends.

Unfortunately, gaining an hour now means losing an hour in the spring. At that time, the news isn’t so good—the downsides of the spring forward are often more severe than the benefits of falling back. Likely due to lack of sleep, the beginning of daylight saving has effects such as:

  • In the same study mentioned above, that one day of reduced heart attacks was outweighed by at least three days of increased heart attacks when DST began again.
  • Workplace injuries increase on Mondays after daylight saving starts. There is no corresponding decrease when it ends.
  • “Cyberloafing”—browsing the Internet to slack off at work—increases the Monday after DST.
  • That can affect the stock market. One paper estimated a $31 billion one-day loss on the major stock exchanges in the U.S. on that fateful Monday.
  • Many of these effects can be explained by a general reduction in cognitive performance. Another study found that SAT scores in regions where daylight saving is imposed are severely impaired. This led the authors to conclude: “Starkly expressed, DST appears to cause brain damage." They suggest that governments should rethink daylight saving, “unless those government officials have already suffered the cognitive debilitation of too much DST.”

If DST is terrible for the brain (not to mention health and the economy) why do we continue doing it every year? There are some arguments in favour of the shift, such as reduced traffic collisions when there is more daylight during high-traffic times. Perhaps more importantly: it can be hard to motivate change when these high-level negative trends are hidden by the fact that individual people rarely notice or measure the effects.

* Or vice versa if you’re in the Southern Hemisphere.

This post was written by Mike Battista, a staff scientist at Creyos (formerly Cambridge Brain Sciences), who is, despite all this, looking forward to an extra hour of sleep this weekend.


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