What happens to your memories while you sleep?

As you slumber, the brain is a whir of activity sorting and storing your memories. How does it know which to choose, and how can you game the system?

By Catherine de Lange

There is an old wives’ tale that putting your revision notes under your pillow the night before an exam will make you remember more. That might be stretching the truth, but there could be something in it – you really do learn in your sleep.

You don’t need sleep to create a memory. “But sleep plays a critical role in determining what happens to these newly formed memories,” says Bob Stickgold at Harvard Medical School. Sleep determines what goes into long-term storage. It can also select which parts of a memory to retain. And it links new memories with established networks of remembrances. It discovers patterns and rules, says Stickgold, “and it’s doing this every night, all night long.”

One of the biggest unanswered questions is how the sleeping brain knows which memories to strengthen, and which to ignore. “We don’t know either the algorithms the brain uses to make these decisions, or how they are implemented,” says Stickgold.

What we do know is that sleep is special. “During slow-wave sleep, there is this release, a kind of beautiful set of interactions between different brain areas, that is specialised, and it looks different than what we see during awake periods,” says Anna Schapiro, also at Harvard Medical School. There is conversation between regions key to memory, including the hippocampus, where recent memories are stored, and the cortex, where long-term memories end up.