Go to bed! As a parental command, you defied it. Now, older though no wiser, it is your body that is issuing the order and you’re still turning a deaf ear. Our species hasn’t shuffled to the Circadian beat for a century; we are a nation of sleep debtors who start running up the bill in junior high, borrowing more than we can ever hope to repay. But pay we will.

Though much of the Morphean landscape—what should be 1/3 of our lives—remains incognito, gathering evidence indicates that every hour cheated from the backside of the clock gets tallied, Scroogelike, by our wearying bodies, with nasty, system-wide consequences.

Sacrifice a couple hours of sleep and the first thing to go is intricate thinking.  The brain uses sleeptime to replace energy molecules, adenosine triphosphates, expended during waking hours, and starts closing shop until fed by slumber. As the more exquisite cognitive functions erode, so too your ability to adapt to shifting circumstances. Ergo, avoid anything that benefits from snappy improvisation: bebop, rock climbing, lying to your teacher.

Fritter away more sleep—nothing dramatic, say eight hours over a week’s time—and your alertness will suffer, speed and reaction time diminish, recovery from exercise attenuates, whatever drives us to endure prolonged activity gets sapped, our mood sours (and, really, why stay up if only to be pissed off?). Sleep is being denied its ill-understood but patent restorative work. Then your endocrine system turns ugly.

Even the modestly sleep-deprived, say four hours over one night, display hormones run amok. The stress hormone cortisol, which prepares us each morning for that day’s curveballs, is found to be jazzily at work when it should be tucked in for the night. Meanwhile, production of prolactin, overseer of the immunological system, and human growth hormone, which maintains muscle and connective tissues, decreases markedly. Without these hormones up to snuff, muscle degenerates, fat tissue accrues, and we court opportunistic infection—a three-part disharmony that sounds an awful lot like aging, yet another condition characterized by sleep loss.

Neither null state nor luxury, sleep is as critical as diet and exercise to our bien-etre. An undisturbed eight hours is a good target for most everyone. “You can fit the differences among virtually all adults in about an hour’s span,” says Brown University chronobiologist Mary Carskadon.

Imagine life without an alarm clock. Forget rising at 5 in the morning to go to the gym. Sleep in. You have the doctor’s blessing.

(from Outside magazine)