So what is happening during those precious hours when we're asleep? Is it really a time of restoration for our brains? And is it possible that it's more than that?
What happens in our brains while we're asleep is a question neuroscientist Penelope Lewis is trying to answer.
Lewis directs the Sleep and Memory Lab at the University of Manchester in England. In her new book, The Secret World of Sleep: The Surprising Science of the Mind at Rest, she discusses how sleep makes memory stronger, provides what she terms "spring cleaning" for the brain, and plays a role in depression.
How Sleep Strengthens Memory
Have you ever had an experience in which you're practicing a particular skill, say playing the piano, a golf swing or a new language? You go to bed tired and wake up to find you seem to have improved. You're able to play the piano piece more smoothly, your golf swing has straightened out or the words in the new language come more easily.
What happens when we sleep, Lewis says in an interview on National Public Radio, is that "the neural responses in your brain that are associated with things you've recently experienced are spontaneously replayed, or, we say, 'reactivated' while you're asleep."
And it is this reactivation that occurs during sleep that strengthens our memory. Our brains are, in effect, practicing while we sleep. Take playing the piano, for example. If during the day you moved your fingers to play a particular piece, the associated motor areas of your brain would become active while you sleep.
According to Lewis, neuroscientific inquiry is beginning to determine which skills improve with sleep and how such consolidation occurs in the brain.
What is "Spring Cleaning" for the Brain?
During our waking hours, we encounter a huge array of sensory information. We are constantly hearing, seeing, smelling, feeling and tasting. And we have thoughts and feelings about this wide range of sensory experiences. For example, on any given day, by breakfast you may have heard your alarm and thought to yourself that it's too loud; felt the warmth of the shower and noted a need to buy more soap; scrubbed the tiles or fixed a dripping faucet. You may have noted the feel of certain clothing and had any number of thoughts about your closet and the clothes available to you, all while listening to the radio, taking in news stories, noting a favorite song and mentally rehearsing for what the day ahead entails.
The point is that we are bombarded with sensory information all day long. According to Lewis in the interview on National Public Radio, "while we're busy doing things, experiencing things, seeing things, hearing things, learning things, processing different kinds of information, the connections between neurons in the brain get strengthened because they're trying to retain all of this information. And an awful lot of it is garbage; it's stuff you don't want to remember or don't care about — what you had for breakfast, or the color of a stain on the cover of a book or something. It's really not useful or interesting."
If we don't filter out some of this information, our brains become overloaded. We must have a way to sort through the information we receive during the day, storing and consolidating what is important and letting the rest go. That process, Lewis says, happens during sleep. During the deep stage of what is called slow-wave sleep, synapses get downscaled again, according to Lewis. This allows us to recall the salient aspects of our day, without being overwhelmed by unimportant details.
Sleep and Depression
Sleep, specifically REM sleep when we dream, has recently been connected to depressive episodes. REM sleep is associated with strengthening emotional memories. When someone is depressed, their emotional experiences during the day tend to be sad, miserable and depressed.
According to Hornung and colleagues, people who are depressed show drastic increases in REM. These depressed people also are biased toward negative memories. Recent research on antidepressant drugs has found that antidepressants correct the imbalance of REM sleep, while at the same time improving mood. The correlation between improved mood and suppression of REM sleep suggests that negative memories strengthened during REM sleep may play a role in maintaining depression.
We spend as much as a third of our lives asleep. Understanding what happens during sleep, how our brain processes information during sleep and how it serves a restorative function can help us to improve how we learn and determine how to treat emotional and cognitive problems that may be related to sleep.
