How the Sleep Process “Cleans Up” the Brain

Everybody likes sleep. Some of us get more than others. Sleep rests our bodies and restores our energy. The chemical processes that occur during sleep clear out the brain’s cellular waste.

A paper published last week in Science finds that we sleep to forget some things we took in during the day.  To learn, we grow connections (synapses) between neurons in the brain. Through these bridges, neurons send signals to each other quickly, storing new memories in the network.

A 2003 study found that synapses grow so much during the day that it overloads the system, creating noisy circuits. Sleeping clears out the “busy signal” by pruning the excess connections. During deep sleep, the electric waves in the brain slow down due to shrinking synapses.

The same researchers from the 2003 study tested this theory in 2013 studying the synapses in mice brains. The synapses in the brains of sleeping mice were 18% smaller than ones that were awake. The number of surface proteins dropped during sleep. The research finds that sleepiness triggers neurons to make the Homer1A protein and sends it to synapses. This protein sets the “pruning” mechanism in action. Mice injected with a chemical to block the neurons from getting pruned had trouble with fuzzy memories the following day, compared with mice who were not injected. One fifth of the synapses did not change during the pruning process, as they may load well-established memories into the long term bank where they will not be tampered with, like a protected hard drive that cannot be erased.

It is hard to tell if the brain changes at night were caused by sleep or by the biological clock. This pruning could be a function of sleep or the purpose of sleep. The new findings should prompt a look at what sleeping drugs do to the brain. They may interfere with the pruning required for forming memories. Future sleep medicines need to target the molecules directly involved in sleep so synapses get properly pruned.

Zimmer, Carl. “The Purpose of Sleep? To Forget, Scientists Say.” Science. The New York Times, 2 Feb 2017. Web. 5 Feb 2017.

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