Without Sleep, New Learning Just Bounces Off Your Brain
Deep sleep moves yesterday's memories to long-term storage and clears space for tomorrow's.
Talks at Google · Why We Sleep: Science of Sleep & Dreams | Matthew Walker | Talks at GoogleStay up all night to study and you will learn about 40 percent less the next day. Matthew Walker, a sleep scientist at UC Berkeley, tested this by letting one group of healthy adults sleep normally overnight while keeping another group awake all night in the laboratory, with no naps and no caffeine.
The next day, both groups tried to memorize a list of new facts inside a brain scanner. The sleep deprived group was about 40 percent worse at forming new memories. Walker said it was "the difference between acing an exam and failing it miserably."
The problem shows up in the hippocampus, a small region found on both sides of the brain. Think of it as the brain's information inbox, a short-term reservoir that receives new memory files and holds onto them. In well-rested people, it showed strong activity tied to learning. In the sleepless people, there was almost no signal at all, as if the inbox had been shut down and every incoming file was being bounced.
Walker compared it to the character in the film Memento, who can no longer make new memories after damage to that same structure. Walker said losing sleep shuts down that same ability to learn new things.
Sleep fixes the problem with deep sleep. Electrodes placed across the scalp show big, powerful brain waves during the deepest stages of sleep, with rapid bursts of electrical activity riding on top called sleep spindles. Together they act like a file transfer mechanism.
They shift memories from the short-term reservoir to the cortex for durable safekeeping, the big, wrinkled tissue that sits at the top of the brain. That transfer does two things. It protects yesterday's memories so you remember rather than forget, and it clears out the inbox, almost like moving files off a USB stick. When you wake up, the brain is like a dry sponge again, ready to soak up new information. Without that overnight reset, it stays waterlogged, and there is nowhere for new learning to go.
Questions people ask
Does sleep after learning matter too, or only sleep before?
Both matter. Sleep before learning clears space in the hippocampus so you can take in new information, and sleep after learning moves that information to the cortex for long-term storage. Walker describes the after part as hitting the Save button so new facts are future proofed rather than forgotten.
How much sleep do I need for this memory benefit?
In the question session, Walker pointed to the World Health Organization recommendation of about eight hours for the average adult, with a range of seven to nine hours. In his learning experiment, the well-rested group got a full night of sleep, while one sleepless night left the other group about 40 percent worse at learning.
Can coffee or a short nap make up for a sleepless night of studying?
That experiment did not test a rescue, because both groups went without naps and caffeine. As general background, caffeine can make you feel more alert for a while, but feeling awake is not the same as restoring the deep-sleep activity that shifts memories and clears the inbox.