A drop in body temperature near bedtime triggers the subjective sense that is's time to go to sleep. Responding promptly to this internal signal may help you fall asleep faster and sleep more restfully, according to a report in the current issue of the journal Sleep.
Insomniacs Can Benefit
Making a special effort to cool down before bedtime may be of particular benefit to insomniacs, say researchers Patricia Murphy, Ph.D., and Scott Campbell, Ph.D., of the Laboratory of Human Chronobiology at the New York Hospital/Cornell Medical Center in White Plains, New York.
Body Temperature Varies
Body temperature, contrary to the common belief, is not uniformly 98.6°F. That is merely an average. Temperature cycles from about 1 degree below to 1 degree above this average over the course of the day. For healthy young adults who sleep at night, body temperature usually is lowest around 4 to 5 a.m. Most sleep episodes occur in a window from about 6 hours before the daily low to about 2 hours after it.
Sleep specialist have long debated whether the nighttime drop in temperature induces sleep or follows it. One theory is that is simply the result of lying down and curtailing physical activity.
Study Subjects Monitored for Body Temperature
To investigate this question, Drs. Murphy and Campbell recruited 21 men and 23 women, aged 19 to 82. All of the subjects were healthy and ordinarily slept between 6 and 9 hours at night. For the study, the participants had their sleep monitored for 2 nights, the first to facilitate adaptation and the second to serve as a baseline. Then they spent 3 consecutive days and nights in special studio apartments isolated from all time cues. During the study, they wore rectal thermometers continously to provide a minute-by-minute record of their body temperature.
Study Subjects Discouraged From Physical Activity
They were encouraged to eat and sleep whenever they wanted and told specifically not to try to overcome bouts of sleepiness. To boost compliance, the researchers gave them only a deck of cards, a jigsaw puzzle and limited reading material. They could stretch but not engage in any other exercise, and they were discouraged from doing any physical activity that might keep them awake. They were not allowed to take showers and wore comfortable, pajama-like clothing throughout the 72-hour study period. The lights were kept low. As a result, the participants spent most of the time lounging on the couch or in bed.
At the end of the study, the researchers examined each subject's 72-hour stint in the laboratory, looking for sleep bouts that began between 10 p.m. and 6 a.m. and lasted at least 4 hours. Some 65 sleep episodes met this criteria. The researchers then identified the time at which the subjects' body temperature fell most precipitously. This point almost always occurred in the 2 hours before sleep began.
Study Subjects Encouraged to Sleep When Sleepy
In everyday life, Drs. Murphy and Campbell point out, it is easy to disregard the body's readiness for sleep: watching the last innings of a baseball game or reading a good book can serve as a potent distraction. On the night before the subjects began their stay in the time isolation laboratory, the interval between the fall in body temperature and the onset of sleep was about 60 minutes. In the laboratory, where subjects were encouraged to go to sleep as soon as they felt sleepy, the interval was only 44 minutes.
Older Subjects Wake More Often
On the pre-study baseline night, older subjects slept worse than younger subjects, waking more often after sleep began. In the time isolation part of the study, this difference nearly disappeared. That suggests, the researchers say, that older people might sleep better if they responded promptly to sensations of sleepiness, which may mean going to sleep a little earlier than they customarily do. "There's a trade-off, however," Dr. Campbell points out. "While they may sleep more soundly, they also may awaken earlier than desired in the morning. They need to decide in advance what to do if they get up before the rest of the household."
Hot Baths Can Help
People with trouble falling asleep might benefit from taking hot baths about 90 minutes before bedtime, the researchers speculate. When they get out of the bath, body temperature will drop rapidly, and that might help them to fall asleep faster.
Sleep is the journal of the American Sleep Disorders Association and the Sleep Research Society. These independent organizations represent more than 3,500 physicians and other clinical specialists, laboratory scientists, and technicians in pulmonary medicine, neurology, psychiatry, psychology, otolaryngology, internal medicine, pediactrics and other disciplines.
Patricia Murphy and Scott Campbell. Nightime drop in body temerature: a physiological trigger for sleep onset? Sleep, 1997; 20 (6):505-511.
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