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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Minding Your Mind Minding Your Mind

How Sleep Affects Learning and Memory

September 24, 2012

By Michael Craig Miller M.D.

Harvard Medical School

Scientists have long known that getting a good night's sleep helps us learn and remember. Evidence is growing that the sleeping brain actively strengthens memories.

The sleeping brain lays down two kinds of memories.

  • Declarative memory is memory for information — facts, dates and names.
  • Procedural memory is what allows us to play a musical instrument, ride a bicycle or type on a keyboard.

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The Pattern of Learning

Sleep scientists think that different parts of the sleep cycle influence these two types of memory. But there is a consistent pattern. You:

  • Learn something new during the day
  • Strengthen what you have learned during a good night's sleep
  • Remember or perform the task better in the morning

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Stages of Sleep

The two main kinds of sleep are:

  • Rapid eye movement (REM)
  • Non-REM (NREM)

Scientists identify these stages by measuring the brain's electrical activity using an electroencephalogram (EEG).

NREM has four stages, from shallow to deep (stages 1 to 4). The two deepest stages (3 and 4) are also called slow wave sleep (SWS) because the wave forms on the EEG occur at the lowest frequency.

REM sleep alternates regularly with NREM. The cycle occurs about every 90 minutes. Early in the night, NREM dominates, but toward morning, REM sleep takes up larger parts of the 90-minute cycle.

We dream most vividly during REM sleep — and the most vivid dreams of all occur at the end of the night. In general, NREM sleep promotes declarative memories and REM sleep smooths out procedural memories, but the distinction is not clear-cut.

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How Sleep Helps the Most Challenging Memories

Experiments suggest that the sleeping brain works harder on the things that are most difficult for us to learn when we are awake.

For example, when subjects do a key-tapping exercise, they are able to work quickly through some of the easier subsequences, but suffer a slowdown in the transitions between the easy parts. After a night's sleep, these difficult transitions show the greatest speed boost.

The author of the study, Dr. Robert Stickgold of Harvard Medical School, says, "You learn things in pieces and then sleep smoothes them out."

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Sleep and Creativity

Some propose that sleep promotes creativity and insight, not just straightforward procedural learning.

In one study, subjects were given math puzzles to solve. A group that had slept between sessions did the problems slightly faster compared to a group that had not slept. More impressive, the subjects who slept were more likely to find a hidden solution to the puzzle that required insight, rather than a performance boost.

Some researchers theorize that REM has a special status when it comes to insight. The dreams of REM sleep tend to have connections or associations made by leaps rather than a straight, logical progression. REM sleep may advance the original thinking required for insight. In fact, some experiments show that subjects do better on tests of cognitive flexibility after periods with higher proportions of REM sleep.

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The Role of Hormones

Today's researchers have many ways to study the physiology of sleep, but a simple measure — hormone levels in the blood — may be particularly significant.

The level of the stress hormone cortisol gradually rises through the night. It peaks during the final REM period. Some speculate that cortisol helps the brain's work during this stage of sleep, when dreaming links new experiences with old memories.

This could be an evolved feature of the brain that prepares us to deal more creatively with life's challenges.

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Less Sleep May Affect How Well You Learn

How much can you cut down on sleep without cutting down on learning and memory? Probably not much. The best results seem to require eight or nine solid hours of sleep, including that last REM period. This conclusion should be sobering for high achievers who burn the midnight oil. Common sense, and now science, show that success and satisfaction in learning are the payoff for a full night's sleep.

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Michael Craig Miller, M.D Michael Craig Miller, M.D., is Senior Editor of Mental Health Publishing at Harvard Health Publications. He is an assistant professor of psychiatry at Harvard Medical School. Dr. Miller is in clinical practice at Beth Israel Deaconess Medical Center, where he has been on staff for more than 25 years.

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