Is the Purpose of Sleep to Let Our Brains “Defragment,” Like a Hard Drive?

• Is the Purpose of Sleep to Let Our Brains “Defragment,” Like a Hard Drive?

  • the function of sleep is to reorganize connections and “prune” synapses
  • Memory formation involves a process called long-term potentiation (LTP), which is essentially the strengthening of synaptic connections
  • most of the synapses that strengthen during memory are based on glutamate
  • Glutamate works as a transmitter molecule by opening channels on the cells that receive it. The channels allow calcium into the cells on the receiving end, which activates them, allowing messages to go through. But too much glutamate can cause excess calcium to build up inside the very cell
  • a harmful process called excitotoxicity
  • One function of sleep, according to the theory, is to protect the brain against excitotoxicity or other “synaptic overload” problems by pruning the synapses
  • If the brain is essentially removing the “extra” synaptic strength formed during the previous day, it must do so in a way that preserves the new information. One possible mechanism for this is synaptic scaling.
  • After some of the neural connections into a given cell, or “inputs,” become stronger, then all of the synapses on that cell could be weakened. This would preserve the relative strength of the different inputs, while keeping the total inputs constant.
  • sleep may serve to reorganize and reconsolidate memories
  • This theory is specifically about slow-wave sleep (SWS). It doesn’t try to explain rapid eye movement (REM) sleep, when dreams happen. Interestingly, some animals do not have REM, but they all have SWS. In some animals, like dolphins, only one side of the brain has it at a time, which is strong evidence that SWS, but not REM, is vital for life.)
  • synaptic strength and density is increased in sleep-deprived animals, compared to recently rested ones.
  • The results showed a steady increase in brain excitability with increasing time spent awake. Sleep put this back to normal—mostly.
  • consistent with the idea that synapses become steadily stronger during wakefulness, and are pruned during sleep.

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