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THERE is an old wives鈥� 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 鈥� you really do learn in your sleep.

You don鈥檛 need sleep to create a memory. 鈥淏ut sleep plays a critical role in determining what happens to these newly formed memories,鈥� 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, 鈥渁nd it鈥檚 doing this every night, all night long.鈥�

One of the biggest unanswered questions is how the sleeping brain knows which memories to strengthen, and which to ignore. 鈥淲e don鈥檛 know either the algorithms the brain uses to make these decisions, or how they are implemented,鈥� says Stickgold.

What we do know is that sleep is special. 鈥淒uring 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,鈥� 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. This chatter might be allowing the cortex to pull out and save important information from new memories.

We don鈥檛 need to recall everything that happened in a day, and sleep favours certain types of memory. It homes in on information that might be useful at a later date, and puts it into longer-term storage. Schapiro has found, for example, that merely telling people they will be tested on certain material helps them remember more of it after sleep.

Memories with an emotional component also get preferential treatment 鈥� especially negative emotions. That makes sense from an evolutionary perspective if we are to remember our mistakes and so increase our chances of survival.

However, there are also hints that sleep might help to modulate emotional memories. 鈥淚f you have a memory that was really intense, sleep will help to preserve the memory, but decrease the emotionality,鈥� says Schapiro.

鈥淪leep will help to preserve a really intense memory, but decrease the emotionality鈥�

This could be crucial for our mental health. 鈥淧ost traumatic stress disorder might actually be a direct consequence of failures of those sleep-dependent processes that weaken the intensity of emotional responses to memories,鈥� says Stickgold.

It could also help explain why getting too little sleep is so bad for you. Negative memories become dominant over neutral and positive ones, for a start. We end up less wise too, says Stickgold. 鈥淲e remember facts and events, but don鈥檛 manage to figure out what they really mean for us and our future.鈥�

And what about advice for anyone with exams on the horizon? 鈥淚t鈥檚 much better to go to sleep between studying and taking a test than to stay awake all night studying,鈥� says Schapiro. So put those notes under your pillow and get some shut-eye. Your brain should do the rest.

This article appeared in print under the headline 鈥淲hat happens to your memories when you sleep?鈥�