Trimmed during sleep

Cut in half, every night

Key ideaTurn every link down evenly: memories stay, and room opens up

While you are awake, links in the brain keep getting stronger. Left alone, they would fill up, with no room left to store more. So one idea is that during sleep, all links are turned down evenly. On screen, every link is cut in half and then we recall the tree — and the picture comes back unchanged. The memory stays, and there is room for tomorrow.

Why?If every link is turned down by the same amount, which links are stronger and which are weaker stays the same, so the stored pictures stay too. What you gain is room to store new things. Some explain the clear head and fresh learning after a good sleep this way.

Go furtherThe idea comes from the researchers Giulio Tononi and Chiara Cirelli in the US and is called the synaptic homeostasis hypothesis. It is still a hypothesis, and sleep is thought to do several things for memory at once.