Scientists are finding new evidence that a good night's rest plays a crucial role in cementing memories formed during the day.
One new study has identified a brain region involved, along with the hippocampus, in creating memories of the day's activities during sleep. Another study suggests melatonin, a hormone involved in regulating our day-night cycle, or "circadian rhythm," acts to suppress the formation of new memories as bedtime nears, perhaps in an effort to give memories made earlier in the day a chance to be prepared for long-term storage.
Both studies are detailed in the Nov. 16 issue of the journal Science.
Prepping for storage
In 1993, scientists learned that the hippocampus "replays" the day's events during sleep. The process appears to be important for consolidating new memories and preparing them for long-term storage in other brain areas.
In one of the two latest studies, David Euston of the University of Arizona and his colleagues found that the medial prefrontal cortex, a brain region implicated in the retrieval of memories from the distant past, was also active during learning and replayed the day's events during sleep. And similar to memory replay in the hippocampus, events were speeded up when reviewed.
Euston's team recorded activity in the medial prefrontal cortices of rats as they ran on a track and afterward while they slept. When the rats were running, brain cells in the medial prefrontal cortex fired off electrical signals in specific patterns over time. The patterns of electrical firing corresponded to memories.
"You see a series of these patterns," Euston said. "You can imagine at point A there'd be one pattern of cells firing, and at point B there's another pattern."
The rats' brains were scanned again as they rested after performing the task. "When the rats go to sleep, we can continue to monitor the activity of the cells, and we look for a re-expression of those same activity patterns," Euston told LiveScience.
Memory fast forward
The researchers found the patterns, but discovered they were being replayed about seven times faster than when the rats were actually performing their tasks.
"In the maze, the rat might take 1.5 seconds to get from point A to point B," Euston said. "When the rat goes to sleep, you see those patterns replaying, and the entire thing takes only 200 milliseconds."
The researchers say the medial prefrontal cortex's fast-forward replay of the day's events could be evidence that our brains can process information much faster when not busy with real-world tasks.
"When you're awake and performing things, the brain has to go at the pace at which your behavior is unfolding," Euston said. "If you're reaching for a cup, the cells in your motor cortex have to be expressing the patterns of activity that will guide your hand to the cup. When you go to sleep you don't have that constraint anymore."
Brain-imaging studies involving people have also shown the medial prefrontal cortex to be active during learning, so the same processes could apply to humans as well, Euston said.
In order to ensure that memory consolidation proceeds smoothly, our brains might have a built-in mechanism that inhibits the formation of new memories as we get closer to bedtime, the second new study finds.
Gregg Roman at the University of Houston in Texas and his colleagues linked the hormone melatonin to the quality of memories formed in zebrafish . They showed that zebrafish trained to perform a task during the day, when melatonin levels are typically low, remembered what they were supposed to do better than if they were trained at night, when levels of the hormone peak.
As further support for melatonin's role in memory, the team found that fish administered with melatonin during the day had trouble forming new memories, and that night training which occurred in the presence of constant lights (which inhibits melatonin secretion) yielded strong memories.
Roman speculates melatonin blocks new memory formation so that older experiences accumulated during the day have a chance to solidify.
Melatonin is an important hormone in every creature from cockroaches to humans, so it's likely the zebrafish findings also apply to humans, Roman said.
So does that mean learning is best done during the day and not at night?
Maybe, Roman said, but he points out that the human memory system is much more complex than that of the zebrafish, and while melatonin should inhibit memory formation at night in us, its effects will be buffered by other hormones and other brain components.
"I would presuppose that learning could occur at night in humans," Roman said. "We have a much higher capacity for learning than zebrafish."
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