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September 3, 2007

Volume 85, Number 36

Web Exclusive

Sleep Anchors Memory

Aided by dramatic swings in neurochemicals, different stages of sleep help preserve different types of memory

Sophie L. Rovner

You'll never again take sleep for granted once you talk to Matthew P. Walker, a psychologist at Harvard Medical School's sleep and neuroimaging lab. "Sleep plays an important role in processing memories," he declares.

Say you're taking piano lessons and you learn how to play a scale. The next day, you'll find that "sleep has enhanced the information that you learned, so you're 20-40% better in performing those motor skills than you were the day before," Walker says. "Your brain has continued to learn in the absence of any further practice, which is quite magical."

The window of time for that improvement is limited, however. If you're a college student and you pull an all-nighter after the piano lesson, you lose out on the memory enhancement permanently, even if you sleep the next night, Walker says. "It's not practice that makes perfect," he says, "but it's practice with a night of sleep that seems to make perfect. If you don't snooze, you lose."

NEUROMODULATORS These compounds fluctuate throughout the night, fine-tuning the impact of the neurotransmitters glutamate and γ-amino butyric acid.

Sleep apparently rearranges memory within the brain. "We presume that it's organized into a more efficient storage location," Walker says. "That means you can recollect that information the next day much better than the day before."

Sleep has this effect only on procedural memories, that is, memories associated with a physical procedure such as playing a piano or riding a bike. These memories are normally used without conscious effort for motor skills that can't readily be described in words. Furthermore, it's only light dreamless sleep that works this magic.

By contrast, deep dreamless sleep consolidates, or crystallizes into long-term memories, newly formed declarative or episodic memories, such as what you had for dinner yesterday or the fact that Paris is the capital of France. Sleep stabilizes these memories and prevents them from decaying over time. Unlike procedural memory, Walker says, declarative memory "very rarely improves." Walker recently found that sufficient sleep prior to learning is also necessary for effectively retaining episodic memory (Nat. Neurosci. 2007, 10, 385).

Emotional episodic memories depend on dreaming sleep for consolidation, Walker says. That dependency could conceivably provide an entr??e for toning down the emotional memories associated with conditions such as posttraumatic stress disorder. "Perhaps we can try and modify sleep neurochemically to prevent amplification of those memories and ameliorate some of the symptoms," he suggests.

The brain is steeped in a neurochemical cocktail that changes during the different stages of sleep, Walker notes. The neurotransmitter glutamate revs up the brain, causing neural transmission, whereas the neurotransmitter γ-aminobutyric acid suppresses neural activity. The impact of these two chemicals is fine-tuned by neuromodulators including acetylcholine, serotonin, and noradrenaline (also known as norepinephrine).

"When we're awake, these three neuromodulators are all swilling around the brain in fairly high concentrations," Walker says. "But as we start to fall asleep, they start to decrease in concentration." Acetylcholine drops to its lowest levels during dreamless sleep. During the transition to dreaming sleep, noradrenaline and serotonin levels remain low, but the acetylcholine level begins to rise, reaching twice the waking level in some parts of the brain. The body cycles several times between dreaming and nondreaming sleep throughout the night.

The three neuromodulators are probably intricately involved in the impact of sleep on memory, Walker notes, though for now, that's just an educated guess. "People know a lot about the brain chemistry of sleep, and people know a lot about the brain chemistry of memory," he says. "But very few people have started to try and put those two fields together to understand why sleep seems to offer these benefits to memory. We're really just starting to scratch the surface now."

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