You may seem completely out of it as you snooze, but deep within, your body works overtime. Your body alternates between 90-minute to 2-hour cycles of deep, or slow-wave, sleep and REM, or rapid eye movement sleep. During deep sleep, organs, bones, and tissues are repaired, while during REM, emotions and memories are processed. For the greatest benefit, you need at least eight hours of sleep with a high percentage of deep sleep in the first two hours and mostly REM sleep in the last two hours, says James B. Maas, Ph.D., a Cornell University psychology professor in Ithaca, New York.
Throughout the night, your body tunes itself up, recharging your batteries, resetting your thermostats, and topping your fluids so you can operate at your peak. Your brain is the conductor, refurbishing your worn organs, burning calories, releasing hormones (including growth hormone, which builds muscle), and processing and storing the day's memories and lessons. All of your body's parts play their roles with the well-timed precision of a symphony orchestra. Instead of a major finale, this concert ends when your biological clock registers that you've paid off any sleep debt accumulated to that point.
Interrupt the performance too early and you lose your rhythm. "The consequences are crankiness; slowed reaction time; hampered creativity; inability to remember, analyze, do math, or make decisions; and most of all, daytime drowsiness," says Maas, author of the book Power Sleep (Harper Collins, 1999). "Sleep deprivation makes you stupid."
You might drop your dishes in the clothes dryer and not realize it until you turn the machine on and they shatter. You may fall asleep at a red light, waking only when the driver behind you honks. Or you could fall asleep during sex -- not exactly conducive to romance. All have happened to sleep experts' patients. "The real danger is that we don't realize we're tired," says David Dinges, Ph.D., a sleep researcher at the University of Pennsylvania in Philadelphia. "We'll go about our business as if we were capable."
To return to full capacity, we've got to repay our sleep deficit. Indeed, Dinges says, sleep debt is much like a financial one. Once you've robbed your dozing bank, you've got to refill it with the same amount, ideally by oversleeping for days in a row.
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