The Sleep-Memory Connection

How getting more shut-eye helps memory and learning.

By: Phil Scott | August 28, 2006

REMEMBER BEING THE BLEARY-EYED STUDENT who pulled an all-nighter to cram for a test? You probably smile at that behavior now because you know that approach usually doesn’t work so well. Through experience, you’ve learned that it takes a good night’s sleep to keep the brain alert and functioning smoothly. No matter how much you love Jane Austen, Sense and Sensibility will put you to sleep if you’ve had only a couple of hours’ rest the night before.

New Connections. Now science is telling us what’s behind the lessons we’ve learned in life: According to researchers, sleep is necessary for the brain to process and consolidate knowledge and for memories to form. During sleep, researchers say, the hippocampus (where memory is stored) becomes highly active and moves knowledge from short-term memory to long-term memory. In a study published in the April 2001 issue of Neuron, Marcos Frank, PhD, a neuroscientist at the University of Pennsylvania, analyzed brain plasticity, that is, the brain’s ability to form new synapses and connections between brain cells.

Four groups of young cats whose brains were still developing were used in the experiment. The cats were exposed to light with one eye covered. (Covering the eye causes changes in the cats’ visual cortex, and the study was to determine if sleep enhances that process.) The first group was studied right after being exposed to the light. The second group was allowed to rest in the dark for 6 hours. The third group was deprived of sleep in the dark, and the fourth was deprived of sleep in the light. In analyzing the cat brains, the scientists discovered that in the second group—the one allowed to sleep—the area of the brain that processes information had doubled in size relative to the first group. The cats who were not allowed to sleep showed no changes in brain plasticity. Frank’s conclusion: The brain needs sleep to learn.

Your Brain at Night. All mammals experience two types of sleep. In REM (rapid-eye-movement) sleep, there is paralysis of all muscles, changes in the heart rate and respiration, and rapid back-and-forth eye movement (almost as if your eyes were following the action in your dreams). REM sleep is, in fact, when the most vivid dreams occur. “When you observe a brain during REM sleep, it resembles what it looks like when you’re awake,” Frank explains. During non-REM (NREM) sleep, however, brain activity diminishes, and the subject relaxes completely. In Frank’s cat study, the subjects’ brain plasticity grew with greater amounts of NREM sleep. His experiments seem to indicate that NREM sleep is more important to plasticity than REM sleep, so he’s studying the subject further. Research has already shown that the brain behaves the same during NREM as it does while it’s at rest. So mere rest may perform the same function as sleep.

Scientists say sleep has the biggest effects on procedural learning, such as how a dancer would learn a new step, or how you would learn to fly an airplane or play a musical instrument. That’s in contrast to declarative memory—recalling facts such as “this is a tree,” “this is my friend John,” or “this is what I ate for lunch 6 days ago.” No amount of sleep can help make declarative memory sharper.

When You’re Older. Although Frank analyzed the brain plasticity of young cats all of the same age, he says that there is evidence that it is highly likely that similar processes occur throughout an animal’s lifespan. Trouble is, the older you get, the less sleep you seem to get, right? “In older people we see that REM sleep can drop almost to zero,” explains psychologist Rubin Naiman, PhD, clinical assistant professor of medicine at the University of Arizona’s Program in Integrative Medicine and author of Healing Night: The Science and Spirit of Sleeping, Dreaming, and Awakening. It’s not so much that we need less sleep, but that circumstances conspire to allow us to sleep less. In fact, “as people age, possible cognitive deterioration occurs because their sleep is interrupted,” Frank says.

What’s more, our society seems to believe that sleep is overrated. “We are living in a culture that dismisses dreams,” says Naiman. “We need to begin to recognize the value of dreaming.” Even that dream in which you show up at school in your underwear, unprepared for the quantum physics exam—hey, it’s only a dream.

Phil Scott is the author of Hemingway's Hurricane and has written for Scientific American and New Scientist. This article was published in NRTA Live & Learn, Summer 2006.

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