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Here’s what you’ll learn when you read this story:
We have evolved a sleep cycle that goes from non-REM to REM sleep and back again, but until now, the biological function of this oscillation was unknown.
Researchers have found that non-REM sleep actually helps strengthen memories, while REM sleep cleans them up and prevents them from overlapping.
This process is associated with a neurotransmitter whose low levels during non-REM sleep allow certain neurons to encode memories.
Ever notice how a lack of sleep can leave you somewhat dazed and struggling to remember things? There’s a neurobiological reason for that.
Humans have evolved a sleep cycle that segues from non-REM (NREM) to REM sleep and back again throughout the night. NREM sleep is up first, and during this phase, everything slows down as neurons slow down their firing—body temperatures, breathing, eye movement, heart rate, you name it. This is a period of low brain activity that starts out hazy and sends us off into a deep (and often dreamless) sleep.
Levels of brain activity rise during the transition from NREM to REM sleep. As neurons start firing faster, our eyes move back and forth behind our eyelids (which where the “rapid eye movement” moniker comes from) and our brains become dream projectors. (While it is possible to dream during NREM sleep, REM sleep is when we experience some of our most hyperrealistic dreams.) Once this stage is complete, the brain will then transition back to NREM sleep, and the cycle restarts.
Until now, scientists could not figure out what kind of biological function this sleep cycle had. But using computer models and observing mice, Sara Aton and Michal Zochowski (biologists from the University of Michigan) have developed a theory. NREM sleep may seem like shutdown mode, but it actually helps strengthen memories. REM sleep, then, takes those memories and keeps them in check, keeping them distinct down to the details and preventing overlap between any that are unrelated. This processing of memories needs to happen through a sleep cycle that always starts with NREM sleep—if it went in reverse, memories would be erased.
“NREM drives expansion of memory traces and subsequent REM maintains optimal segregation of traces when multiple memories are being consolidated,” Aton and Zochowski said in a study recently published in the journal PLOS Computational Biology. “The predictions indicate a critical role of REM-based pattern separation during storage of multiple correlated memories in terms of time and content.”
The hippocampus is located in the temporal lobe and carries out functions that help process and retain memories. Previous studies have found that sleep deprivation has a detrimental effect on this part of the brain, resulting in chemical signals that disrupt how memories are consolidated.
By studying hippocampus function in mice that received fear conditioning while still awake, Aton, Zochowski and their research team were able to see which parts of this brain region were activated throughout their sleep cycle. When mice were moved to a different enclosure than the one they were used to, they were given electric shocks to their feet. They were then returned to their regular enclosures. This admittedly isn’t the most pleasant way to create a memory, but it is a potent one. Monitoring the sleeping mice revealed that (as opposed to the team’s control population) the brains of shocked mice showed an increase in functions which strengthened memories.
It is not yet possible to observe every individual human neuron firing. But that was where computer models came in, and the results aligned with what happened to the mice. Simulating what happened to individual neurons gave insight into chemical changes that occur as the sleep cycle keeps repeating itself.
The researchers found that levels of the neurotransmitter acetylcholine matter when it comes to solidifying memories. Acetylcholine factors into voluntary muscle movements and also promotes REM sleep. Low levels of acetylcholine during NREM sleep mean that inhibitory neurons (which help induce sleep) are less active, allowing engram neurons (which encode and retrieve memories) to fire more and strengthen memories. When NREM sleep cycles back to REM sleep, acetylcholine levels rise again, suppressing that firing in order to clean up those memories and keep unrelated aspects of them from overlapping.
“The observed reduction of overlap between the representations during REM maybe also important for preventing catastrophic forgetting (i.e. overwriting old memories with new ones),” Zochowski and Aton said.
Point taken. If you want to remember things, remember to get more sleep.
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