"The results were surprising," said Christian Guilleminault, MD, professor of psychiatry and behavioral sciences at the School of Medicine and lead author of the paper that appears in the May issue of Sleep Medicine. "We had suspected that the more sleep-restricted the participants were, the sleepier they would be - regardless of when they went to bed. That's not exactly what we found."
It is well known that a cumulative reduction in nightly sleep results in decreased daytime functioning and a lower quality of life. Many factors contribute to the ways individuals tolerate sleep restriction, but the impact on the exact time that sleep took place on next-day behavior had not previously been studied.
"The fundamental question we wanted to answer focused on the time of sleep," said Guilleminault. "If people can sleep for only a short period of time, what time should they sleep?"
The study involved eight men ranging in age from 18 to 25. The researchers monitored the participants' sleep and collected baseline data as they slept for 8.5 hours for two nights. The men were then split into two groups: participants in one group slept from 10:30 p.m. to 2:30 a.m. for seven nights; the other group from 2:15 a.m. to 6:15 a.m.
The participants spent the week in the laboratory where researchers tracked their behavior and wakefulness through a series of tests, including the "maintenance of wakefulness" test, a series of daytime nap studies during which a participant is asked to stay awake and not fall asleep; memory tests; and a driving simulator. Blood tests were also done.
Not surprisingly, the researchers found that sleep restriction affected all participants. Results of the wakefulness tests taken the day after 8.5 hours of sleep differed greatly from results on the last day of sleep deprivation. But the results also differed between the two groups, showing that the timing of sleep may have an impact on daytime function.
The early morning sleep group's score on the wakefulness test was significantly better than the late-night sleep group, indicating that early morning sleepers overall were more tolerant of sleep restriction. In addition, the researchers found that participants in the early morning sleep group had better rates of sleep efficiency (the percentage of time spent sleeping in the four-hour window) and sleep latency (the amount of time spent falling asleep).
Data from other tests is still being evaluated, but Guilleminault said these preliminary findings warrant further studies of sleep placement. Such studies could have particular importance to members of the military or other professions where sleep restriction is common, he said.
Aside from the impact of sleep placement, Guilleminault said the most striking part of the study was the great difference in individual responses to sleep deprivation. He noted that one participant in the early morning sleep group was not impacted by sleep deprivation until the sixth day of the study; he functioned in a "borderline normal" way for the first five days. Another participant, this one in the late-night sleep group, reacted so poorly to his new schedule that he developed severe insomnia.
Guilleminault said the unpredictability of a person's response to sleep deprivation is a good reason for people to be cautious about limiting their amount of sleep. "What we show is that everyone becomes impaired by sleep deprivation, and that abnormal responses will be triggered in some people," he said. "Many people stay up all night to study for a final or to finish a big project for the boss, but this sleep restriction can be very detrimental."
Among the researchers' other findings was that the secretion of leptin, a hormone that regulates appetite, decreased significantly during the period of sleep deprivation. Decreases in leptin levels are associated with increased appetite and possibly weight gain, and Guilleminault said further studies should focus on a possible link between sleep deprivation and obesity.
Guilleminault's Stanford collaborators on this study include Nelson Powell, MD; Sandra Martinez, MD; Clete Kushida, MD, PhD; Luciana Palombini, MD; and Pierre Philip, MD. Tifenn Raffray, MD, now at Paris University's medical school, also contributed. The study was funded by the Sleep Education and Research Foundation of Palo Alto.
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