Sleep stages and memory

To fully grasp the importance of sleep to cognition, you need to understand what happens in your body and brain during the various stages of sleep, which cycle throughout the night in a fairly predictable pattern. 

Stage 1. You are in between being awake and falling asleep. Your sleep at this stage is light and easily interrupted. 

Stage 2. This is the first stage of true sleep. Your heart rate and breathing slow, and you start to become unaware of your surroundings. 

Stage 3. (slow-wave sleep or deep sleep). In this phase of sleep, your breathing and heart rate have slowed. Your blood pressure has dropped. Your muscles are relaxed. During this phase, your tissues regenerate and your body releases essential hormones. If someone were to try to rouse you now, you'd have a hard time awakening and would feel groggy.

Rapid-eye-movement (REM) sleep. Dreaming occurs in REM sleep, during which your body is paralyzed but your eyes dart back and forth behind closed eyelids. Your blood pressure increases, and your heart rate and breathing speed up to daytime levels. 

Of these phases, the one that is most important for memory is REM. During the heightened brain activity of REM sleep, you process and consolidate new information you've learned. When you first acquire new memories, they're flimsy and easily forgotten. Your brain needs to process them to make them stick.

Sleep helps shuttle information from the brain's temporary storage—the hippocampus—into its permanent memory troves in the frontal cortex. While you rest, you also merge existing information you've learned with new knowledge, a process that is instrumental to problem solving. 

This is why people often recommend that you "sleep on it" when you're dealing with a problem. While you sleep, your brain prioritizes memories—shoring up the more important ones while marking the less useful ones for deletion. 

It takes more sleep than you might expect to cement existing information into memory. According to Robert Stickgold of the Center for Sleep and Cognition, based at Harvard-affiliated Beth Israel Deaconess Medical Center, for every hour you're awake during the day, you need a full half-hour of sleep time to process the new information you've learned. Not surprisingly, he and his colleagues have found that the reverse also holds true: a lack of sleep can impair the ability to remember and make sense of what you've learned. In one of his studies, students who were taught a new task and then deprived of sleep for a night were less able to remember how to do that task for up to three days afterward. 

For now, the takeaway is that it is essential to get plenty of sleep and go through all the stages of sleep.

For more on staying sharp as you age, read  A Guide to Cognitive Fitness , a Special Health Report from Harvard Medical School.

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