What sleep does with your day

In 1953, a young man named Henry Molaison had brain surgery to stop severe seizures. Surgeons removed much of his hippocampus, a curled structure deep on each side of the brain. The seizures eased, and something else broke. Henry could no longer form new memories of facts and events. He could chat with a researcher for an hour, and if she left the room and came back, he would greet her as a stranger.

Many of his older memories were still there. He could also still learn new skills. Over several days he got better at tracing a star while watching his hand in a mirror, and each day he insisted he had never tried it before.

Henry was studied for decades, and his case taught researchers two big things. New memories of facts and events depend on the hippocampus. Older memories, somehow, depend on it much less.

Drag sideways or tap Turn to inspect the schematic, then compare new and older memories. The purple markers show the paired, deep location in simplified form, not the full anatomy.

From fragile to stable

The process that turns a fresh, fragile memory into a lasting one is called consolidation. A new memory starts out as a pattern tied together by the hippocampus. Over the following days, months, and years, it gets reorganized and linked into the rest of what you know. Researchers still argue about how complete that handoff is, especially for vivid personal memories. They agree that a lot of the work happens while you sleep.

Replay in the dark

In the 1990s, scientists recorded neurons in the hippocampus of rats running along a track. Particular cells fired at particular spots, so each run produced a sequence, like notes in a tune. When the rats slept afterward, the same sequences played again, sped up. The sleeping brain seemed to be rehearsing the day.

Press Play to compare the sequence during a run with its faster replay. Use the slider to step through it; replay preserves the order in this simplified example.

People show the effect in behavior. Learn a list of word pairs, then either sleep or stay awake for the same number of hours, and the sleepers usually remember more. Deep slow-wave sleep and short bursts of brain activity called sleep spindles both track how much people keep.

One experiment went further. People learned where fifty objects sat on a screen, each paired with a sound, like a meow for a cat. Then they napped. During deep sleep, researchers quietly played half of the sounds. After waking, people placed those objects more accurately than the others, and none of them reported noticing the sounds.

The effects are real, but they're moderate, and the details of which sleep stage does what are still debated. Sleep also won't teach you anything new. A language podcast playing all night won't leave you speaking French. Sleep works on what you put in while you were awake.

The all-nighter costs twice

Skipping sleep before an exam hurts in two ways. What you studied that day misses its night of consolidation. And the next day, your brain is worse at taking in anything new. In one study, people kept awake all night showed weaker hippocampus activity while learning the next day, and they remembered noticeably less than people who had slept.

So:

• Study, then sleep. Material you review in the evening gets consolidated that same night.
• If the choice is one more late hour of review or a full night's sleep, sleep usually wins.
• Short naps seem to help too, though the evidence for them is thinner.