Wake Time Influences Circadian Clock More Than Bed Time

Yael Waknine

October 18, 2005

June 28, 2005 — A late wake time causes a significantly greater phase delay in the circadian rhythm than does a late bedtime, according to the results of a small prospective study presented at the 19th Annual Meeting of the Associated Professional Sleep Societies in Denver, Colorado.

"Modern society is chronically sleep deprived; a poll by the National Sleep Foundation in 2001 found that about a third of Americans report sleeping six hours or less during weeknights and 17% of people continue to do so through the weekend," Helen Burgess, PhD, lead investigator, told Medscape. "These short sleep episodes are also associated with a reduced dark period, and we were interested in the effect of these short dark episodes on the circadian clock."

Dr. Burgess is the assistant director of the Biological Rhythms Research Laboratory at Rush-Presbyterian-St. Luke's Medical Center in Chicago, Illinois, where she is an assistant professor.

In the study, investigators combined a regular late bedtime with an early and late wake time in 14 healthy subjects (4 females, 10 males; mean age, 28.8 ± 5.2 years) having normal sleep length (bedtime, 00:30 ± 1.2 hour; wake time, 7:57 ± 1.0 hour).

Subjects experienced 14 "short" six-hour nights (normal wake time after only six hours of sleep) followed by 14 "long" nine-hour nights (same late bedtime with a three-hour wake delay). The two-week periods were separated by a six-day span in a counterbalanced design.

"Rather than use a set clock time, we based the sleep time on the subject's mean weekday wake time," Dr. Burgess noted.

Subjects slept at home, and compliance was confirmed with a wrist actigraph and a photosensor. Black plastic was placed over bedroom windows to ensure that sleep time occurred during the dark period (< 1 lux). Subjects came to the laboratory at the end of each two-week period for a dim-light phase assessment during which half-hour saliva samples were collected and assayed for melatonin.

Results showed that the late wake time delayed dim-light melatonin onset (DLMO) by 2.5 ± 0.8 hour (21:36 ± 1.2 to 00:07 ± 1.2 hour; P < .001) and DLMOff by 2.9 ± 1.1 hour (8:16 ± 1.3 to 11:09 ± 1.9 hour; P < .001).

"The phase delay in the circadian clock shifted along almost as much as the three-hour delay in wake time," Dr. Burgess said, noting that findings from an earlier study showed a circadian shift of only about half an hour when the same delay was applied to bedtime.

"Anytime you change the timing of your sleep, you are going to change the rhythm of your circadian clock, but you are going to have a much bigger change when you change your wake time rather than bedtime," said Dr. Burgess. "For example, sleeping in on the weekend will have a significant effect on delaying your circadian clock and make it harder to revert back to normal wake time on Monday."

According to Dr. Burgess, the increased phase delay is most likely due to the absence of bright morning light and may also reflect a decreased response to the stimulus. "We have recently shown that phase advances in response to a set light stimulus (generated by a light box) are reduced after short nights compared with long nights. Phase delays are similarly reduced."

"Short sleep episodes are becoming increasingly common, and the National Sleep Foundation suggests that this is partly due to increased work hours — work hours increase but people do not want to give up their leisure time and therefore delay sleep," Dr. Burgess concluded. "If you have been working long hours, the best way to recover from a sleep deficit is to go to bed earlier rather than sleeping in."

This study was funded by the American Sleep Foundation and a grant from the National Institutes of Health Institute of Heart, Lung, and Blood.

APSS 19th Annual Meeting: Abstract0162. Presented June 23, 2005.

Reviewed by Gary D. Vogin, MD

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