Changing to daylight saving time may give people an hour more of sunlight, but it appears that their internal body clocks never really adjust to the change, German researchers report.

In fact, daylight saving time can cause a significant seasonal disruption that might have other effects on our bodies.

"When you change clocks to daylight saving time, you don't change anything related to sun time," explained lead researcher Till Roenneberg, PhD, of Ludwig-Maximilians-University in Munich. This is one of those human arrogances--that we can do whatever we want as long as we are disciplined. We forget that there is a biological clock that is as old as living organisms, a clock that cannot be fooled. The pure social change of time cannot fool the clock."


People's circadian rhythm--the body's internal clock—follows the sun and changes depending on where you live. It actually changes in four-minute intervals, exactly the time it takes for the sun to cross one line of longitude, Dr. Roenneberg explained.

"The circadian dock does not conform to the social change," Dr. Roenneberg said. "During the winter, there is a beautiful tracking of dawn in human sleep behavior, which is completely and immediately interrupted when daylight saving time is introduced in March," he said.

Daylight saving time may be one cause of what Dr. Roenneberg calls our lack of seasonality. By seasonality, he means that our internal clock is in tune with the natural change in light throughout the year. "This could have long-term effects," he said.

Study #1

Dr. Roenneberg's group collected data on the sleep patterns of 55,000 people in Central Europe. The researchers found that sleep time on days off work when daylight saving time took effect followed the seasonal progression of dawn under standard time, but not under daylight saving time.

Study #2

In another study, Dr. Roenneberg's group looked at the timing of sleep and activity for eight weeks during the change to daylight saving time in 50 people, taking into account each person's natural clock preferences, or "chronotypes," which range from morning larks to night owls.

For both morning larks and night owls, timing for sleep and peak activity easily adjusted when daylight saving time ended in the fall. However, it never adjusted to the return to daylight saving time in spring. This was especially true for night owls-those who stay up late and sleep late.


"If we didn't change to daylight saving time, people would adjust to dawn during the summer and again to dawn in the autumn," said Dr. Roenneberg. "But this natural adjustment is interrupted by daylight saving time," he said.

Expert Commentary

"It is not surprising that when you change our time to respond to something other than the sun and daylight that different chronotypes are going to have a difficult time," said Louis Ptacek, MD, an investigator at the Howard Hughes Medical Institute and director of the Division of Neurogenetics at the University of California, San Francisco.

"Before artificial lighting, humans tended to live much more by the sun cycle," Dr. Ptacek said. "Whereas, now, people stay up all night and turn the lights on, which affects our biological clock. There is no question that we have been changing our clocks long before daylight saving time came along.”

So, it's not surprising that daylight saving time affects our internal clock, Dr. Ptacek said. However, it is no more unnatural than our use of artificial light, he noted.

There is no reason to abandon daylight saving time, Dr. Ptacek added. "There may be societal benefits to daylight saving time, such as saving energy."

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