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Scientists are slowly uncovering why time spent snoozing is time well spent

By Margot Hedlin

Try to remember the exact moment you fell asleep last night. Your eyes closed, your breathing slowed, your heart rate dropped. However, the moment you drifted off is impossible to pinpoint, and when you woke up you likely had no memory of the last eight hours, save a few half-recalled dreams. Yet while you rested, your brain and body worked around the clock as a whole nocturnal ecosystem came to life. Your immune system surged into activity, your hormone levels rose and fell and your brain generated new neurons and strengthened memories.

Deprive yourself of sleep, and the potential results—from impaired driving and a bad memory to mood disorders and obesity—can take years off your life. Researchers continue to learn about what happens to your brain and body as you sleep—and their discoveries just might convince you to get a full night's rest.

This Is Your Brain on Sleep

While you lie tucked under the covers, your brain repeatedly cycles through different stages of sleep: light sleep (where you are easily awakened), deep sleep (a comatose-like stage marked by a drop in body temperature and lack of muscle movement) and REM sleep (short for Rapid Eye Movement, a stage named after the observation that your eyes flicker rapidly as you watch your dreams play out). During these cycles, your brain is not actively receiving information—and it uses this much-needed break to run through a battery of tasks required for processes like thinking and learning.

One major chore your brain undertakes while you sleep is sifting through and storing the memories it intends to keep. A leading theory suggests that during deep sleep, memories are moved from temporary daytime storage into parts of the brain that preserve memories long-term. Your brain then cements those memories in place during REM sleep by reliving experiences of the day: that conversation over dinner, the taste of the Ethiopian food you ate, and the scent of the red wine you sipped play over and over in your mind, strengthening connections between brain cells to encode the memory.

But not all memories get this treatment; your brain is selective about the memories it chooses to keep, reinforcing the memories that will be most useful to you in the future. Trivial memories—the color of the cars you walked by this morning—tend to fade, while more important memories—the name of your new boss—tend to stay. Your memory’s selectivity also helps explain a curious phenomenon: Have you ever “slept on” a problem, only to wake with the answer in the morning? The pruning your memory undergoes at night allows unimportant memories to fall away, leaving the relevant thoughts—and the answer—at the front of your mind.

Sleep also helps your brain prepare to learn by promoting the birth of new brain cells, a process called neurogenesis. Scientists once thought that you were born with all the brain cells you would have for your entire life, but now realize that neurogenesis is crucial for learning throughout life. Mild sleep deprivation slows and even stops the rate of neurogenesis in the part of your brain responsible for forming new memories—which may explain why you have trouble recalling the information you learned in the all-night study session the night before the final exam. Severe sleep deprivation is even worse: Mice kept awake for several days have reduced neurogenesis throughout the whole brain.

While all those new brain cells are great for your memory, they also play a role you might not expect: Neurogenesis is thought to help you manage emotions. This means that chronic sleep deprivation, by hurting brain cell birth, might do more than make you feel cranky the next day. Some scientists think that sleep deprivation might speed the development of mood disorders, like depression. A review of 21 studies on insomnia and depression found that people with insomnia are twice as likely to develop depression as people who don’t have trouble sleeping—a fact that might be due to sleep’s neurogenesis-promoting effects.

Deprive yourself of sleep, and you walk through the day with a fogged mind. Sleep deprivation impairs learning and memory, decreases attention span and makes it difficult to keep your emotions in check. It also reduces alertness and response time, a problem that’s becoming all-too common among those behind the wheel: “Drowsy driving” can be just as dangerous as drunk driving. A person who goes 18 hours without sleep is as impaired behind the wheel as someone with a blood alcohol concentration (BAC) of 0.05, which is just below the legal limit of 0.08; a person who hasn’t slept for 24 hours drives as badly as someone with a BAC of 0.10. The consequences of driving on little sleep can be dire: The National Sleep Foundation estimates that drowsy driving results in 1,550 deaths and 70,000 injuries each year.

The effects of sleep deprivation aren’t just short-term. Each night that you skimp on sleep, you accumulate sleep debt, says David Claman, MD, medical director of the Sleep Disorders Center at the University of California, San Francisco. This means getting one good night’s sleep doesn’t compensate for a week of lousy sleep; your body doesn’t forget the hours of shut-eye it missed. Sleep debt makes you physically and mentally fatigued throughout the day—particularly during the early afternoon ‘siesta’ hours, when your energy levels tend to drop.

Sleep: It Does a Body Good

Sleep is indispensible for learning, mood and a safe hand behind the wheel—but its benefits for the brain are only half the story.

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