Does daylight-saving time mess with your internal clock?

Health and medicine explained.
Nov. 1 2007 7:34 AM

Your Health This Week

Does daylight-saving time mess with your body's internal clock? And more.

Dr. Sydney Spiesel was online on Nov. 1 to chat with readers about this article. Read the transcript.

This week, Dr. Sydney Spiesel discusses the disruptive effect of daylight-saving time on the body's internal rhythms, what happens when you accept a health-related spam offer, and the best ideas for improving the health of the world's poor.

The downside of daylight-saving time?

Illustration by Mark Alan Stamaty. Click image to expand.
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Question: After midnight next Sunday, the clocks where you live will move backward by one hour to shift an hour of daylight from afternoon to morning over the next four winter months. You will share this experience of daylight-saving time with about one-quarter of the people on Earth. What happens to your body's internal clock when it is suddenly reset by society's clock?

Context: The body's daily cycle of activities—the circadian rhythm—determines when we are sleepy and alert, when we want to eat, and even when we produce less urine so our nighttime sleep will be less interrupted. Though there is a spectrum of preferences, from "larks" to "owls," the internal clocks that set our circadian rhythm are mainly regulated by the time the sun rises. (Here's how to figure out which kind of bird you are.) We are not consciously aware of this dependency, and our time of awakening is often affected by external forces, like the need to get to work on time. Chronobiologists, the scientists who study our internal clocks, correct for these effects by comparing awake and asleep times on work days and free days. They have found that the relationship between the arrival of dawn and the midsleep point—the time halfway between the moment you fall asleep and the moment you wake up—remains constant, even as the time of sunrise changes when the length of the day varies with the seasons.

Study: A new German study uses this predictable relationship to study what happens to our internal clocks when the external clocks jump an hour forward or backward. Drawing from a database of 55,000 Central European subjects who submitted daily sleep records, they showed that the normal correlation between dawn and the sleep cycle becomes disrupted during the transition to daylight-saving time.

Findings: In an effort to clarify whether this change was due to the changed clock or to some other phenomenon, the authors zeroed in on the sleep-wake and activity cycles of 50 people during the weeks around the spring and autumn leaps forward and back. They found that the spring institution of daylight-saving time was exactly the moment when the coming of dawn disconnected from the body's sleep-wake cycles. When standard time returned in the autumn, the body's circadian rhythm again linked itself to the time of sunrise.

Conclusion: Practically speaking, what does this mean? If, as some recent research has suggested, sleep and psychiatric illness may be closely tied, perhaps the sleep disruptions associated with time changes might affect the incidence of psychiatric disease during the transition periods. An early study suggested that this was, indeed, the case; more recent research on patients with depression casts doubt on the association. There is clear evidence of a spike in car accidents associated with the spring transition to DST and the fall transition back to standard time. I am tempted to think that disturbances in circadian rhythm are the cause.

Question: Roughly one out of three pieces of e-mail spam is devoted to "medicine"—a nutritional supplement purported to enhance health or quality of life, a cheap prescription order, or, of course, the ubiquitous offer of treatment for erectile dysfunction. Do these sites actually deliver? And if they do, are the goods that arrive pure, safe, and effective?

Study: Peter Gernburd and Alejandro Jadad of the University of Toronto set out to answer these questions by setting up three slightly different Internet e-mail accounts in Canada. They got more than 4,000 unsolicited messages in November 2006, one-third of them health-related. Three weeks later, with some trepidation, the researchers ordered one of every product offered.