Why Do We Sleep?

What does sleep do that being awake doesn't?

Posted Jul 29, 2010

It’s a simple question that you’d think would have a simple answer. We know that sleep has restorative effects on our bodies down to the cellular level, but how it all works from a biological perspective has been somewhat of a mystery.

Now we’ve got more details to help tell the story of just what exactly goes on. New research suggests that during the initial stages of sleep, energy levels increase dramatically in brain regions that are active when we’re awake. The scientists behind this latest study believe that this surge of cellular energy may replenish the brain’s processes that we need to function normally during the day.

“Energy,” by the way, refers to ATP (adenosine triphosphate), which is our body’s chief energy molecule. It’s also the same energy currency in other animals like rats, which were used in the study. The researchers found the following:

  •  ATP levels increased in four key brain regions that are normally active during wakefulness, when the rats were in non-REM sleep.
  • During this time, however, an overall decrease in brain activity occurred.
  • When the animals were awake, ATP levels remained steady.
  • When the rats were gently nudged to stay awake three or six hours past their normal sleep times, there was no increase in ATP levels.

What does all this mean?  

The authors concluded that sleep is necessary for this ATP energy surge, as keeping the rats awake (depriving them of sleep to mimic a sleep deprived human) prevented the surge. The energy increase may power restorative processes absent during wakefulness, because brain cells consume large amounts of energy just to perform daily waking functions.

Indeed, there’s a lot to do when we’re awake. In addition to all the tasks we complete consciously, think about all the things we do practically subconsciously like breathing, think about things we do almost on autopilot: surfing the net, walking the dog, sometimes even driving a familiar route.  All this activity requires—you got it— the ATP energy that appears to be produced by a good night’s sleep. And a good night’s sleep is exactly what the doctor ordered to prepare us for the rigors of our day. No sleep, no energy expended in the brain to help us recover from the day and, in essence, recharge our minds for the next adventures of the next day.

It’s also worth noting that the precise mechanics of the brain also remain a mystery to some degree. There’s no doubt that the brain requires a lot of energy to function as the body’s master of ceremonies. We already know that sleep loss can actually result in brain loss. In fact, the obligations of the brain may be the only reason we need sleep: because without a brain the body isn’t worth much. And it certainly can’t do much.

In addition to food, water, and oxygen, the brain clearly needs sleep. If only sleep-deprivation were as compelling of a sensation as feeling famished or thirsty. We always seem to be able to put off sleep. But eating or drinking, well…

Sweet Dreams,


Michael J. Breus, PhD

The Sleep Doctor™