Why losing sleep changes your memories

Sleep is when your brain does much of the work of turning fresh experiences into stable memories, and a tired brain is also worse at pulling memories back out. So losing sleep hits memory twice: what you learn sets less firmly, and what you recall becomes easier to bend. In some studies, sleep-deprived people even became more likely to remember things that never happened.

A new memory is not finished the moment you form it. Your brain encodes an experience in milliseconds, but turning it into a stable, long-lasting memory can take minutes, days or even years. That slow process is called memory consolidation. In 1924, two researchers, Jenkins and Dallenbach, set out to test the idea that memories simply decay with time. They found that people remembered much more after a period of sleep than after the same amount of time awake.

In rats, you can even watch the night shift at work: during sleep, neurons in the hippocampus replay the firing sequences of experiences from before sleep, several times faster than they happened.

Now the twist. In a 2008 study, people learned lists of words like night, dark and coal, all linked to a missing word: black. Those who were sleep deprived at test time were more likely to falsely remember seeing black. In a 2016 study, sleep-deprived young adults and teenagers were more likely than rested people to absorb misleading information into their memory of an event.

That has real stakes. The 2016 researchers suggested checking the sleep history of eyewitnesses. A tired brain is not only a forgetful brain. It is a more suggestible one, and sometimes a more confident one: in another experiment, sleep loss made people more sure they were right, especially when they were wrong.