Replayed during sleep

The day, replayed at night

Key ideaSleep replays the day and makes memories stick

What happens during the day is first jotted down in a place called the hippocampus. While you sleep, the hippocampus quickly replays the day’s memories. As on screen, the day’s pictures light up again one after another, faster and faster. Through this replaying, memories are thought to move to the brain’s outer layer, the cortex, and become long-lasting.

Why?In 1994, researchers in the US saw that the hippocampus cells that lit up while a rat ran a maze lit up again in the same order while it slept that night. Later studies found that disturbing this replay during sleep made memory worse the next day. That is why staying up all night before a test backfires.

Go furtherThere is one hippocampus deep inside each side of the head, behind the temples; it is named after the seahorse it looks like. Memories moving from the hippocampus to the cortex and becoming solid is called memory consolidation. The screen shows replay as a picture show; in real brains it runs several times faster than the original experience.