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Many patients tell me they have a sleep problem because they wake up in the middle of the night for a time, typically 45 minutes to an hour, but fall uneventfully back to sleep. Curiously, there seems to be no consequence to this "problem." They are unaffected during the day and have plenty of energy and concentration to go about their lives.



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The problem, it seems, is not so much with their sleep as it is with a common and mistaken notion about what constitutes a normal night's sleep.



It's a question that Dr. Thomas Wehr at the National Institute of Mental Health asked himself in the early 1990's. He conducted a landmark experiment in which he placed a group of normal volunteers in 14-hour dark periods each day for a month. He let the subjects sleep as much and as long as they wanted during the experiment.



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By the fourth week, the subjects slept an average of eight hours a night — but not consecutively. Instead, sleep seemed to be concentrated in two blocks. First, subjects tended to lie awake for one to two hours and then fall quickly asleep. Dr. Wehr found that the abrupt onset of sleep was linked to a spike in the hormone melatonin. Melatonin secretion by the brain's pineal gland is switched on by darkness.



After an average of three to five hours of solid sleep, the subjects would awaken and spend an hour or two of peaceful wakefulness before a second three- to five-hour sleep period. Such bimodal sleep has been observed in many other animals and also in humans who live in pre-industrial societies lacking artificial light.



Carol Worthman, an anthropologist at Emory University in Atlanta, has studied the sleep patterns of non-Western populations. From the !Kung hunter-gatherers in Africa to the Swat Pathan herders in Pakistan, Dr. Worthman documented a pattern of communal sleep in which individuals drifted in and out of sleep throughout the night.



She speculates that there may even be an evolutionary advantage to interrupted sleep. "When we lived in open exposed savanna, being solidly asleep leaves us vulnerable to predators."



With artificial light, modern humans have essentially managed to extend their daytime activities late into the night, when all other sensible creatures are busy sleeping.



As a result, we have compressed our natural sleep into artificially short nighttimes, but not all people are so easily tamed by artificial light. Some people, who may just have very strong circadian rhythms, still have this primitive bimodal sleep that they confuse with a sleep disorder.



Add these people to the rest of us who, under the pressures of modern life, often have some trouble falling or staying asleep and there is a large captive audience for drug companies.



Thanks in large part to the meteoric rise in direct-to-consumer advertising, medications like Ambien and Lunesta have become household names and seductive panaceas that millions find hard to resist — even though a majority have no serious sleep problem to repair. If it's any consolation to those of you who are awake in the middle of the night for an hour or so, reading or watching television, you may simply be the most natural sleepers.

I have mentioned this in my very first post here : in a natural state, humans do not sleep a long consecutive bout throughout the night. The natural condition is bimodal - two bouts of sleep interrupted by a short episode of waking in the middle of the night.In today's New York Times, there is an article about this: Sleep Disorder? Wake Up and Smell the Savanna by RICHARD A. FRIEDMAN, M.D.:I have nothing to add, except I can also give you an image I dug up - the original data from Wehr's experiment. See how the sleep is bimodal during the long winter nights and gets compressed during simulated summer: