Sleep deprivation

What it does

To your feelings, your memory, your judgement, and your sense of how you're doing.

Hours awake

• 17–19 hours, Like 0.05%: If you got up at 7 a.m., that's midnight to 2 a.m. On some tests, performance is as bad as at a blood alcohol level of 0.05%, with responses up to 50% slower.
• A full night, Like 0.1%: Stay up longer and performance matches a blood alcohol level of 0.1%.
• The next day, On edge: The amygdala overreacts and anxiety rises.

What it can feel like

• Short fuse: Upsetting things hit harder: the amygdala reacts about 60% more.
• Anxious: Next-day anxiety rises, and even a modestly shorter night than usual predicts more of it.
• Foggy: Attention and memory slip.
• Slower: Reactions get slower and less accurate.
• Run down: You get sick more often, and stress and mood get worse.

You can't feel it

In a two-week study, people sleeping 6 hours or less a night built up thinking deficits equal to up to two nights with no sleep at all. Their sense of how sleepy they were barely changed after the first few days: they didn't notice how impaired they'd become.

• Myth: I'm used to five hours. I'm fine. In fact: On short sleep night after night, people's deficits keep building while their sense of sleepiness levels off. Feeling fine isn't the same as being fine.
• Myth: An all-nighter gives me more time to study. In fact: Without sleep the hippocampus takes in new material poorly, so you remember less of what you studied.
• Myth: Being tired is just being tired. In fact: Seventeen to nineteen hours awake can impair you as much as a blood alcohol level of 0.05%.

What's happening in the brain

An alarm turned up, its brakes weakened, a memory inbox that won't take more, and a night shift that never came. The model lights each one as you read.

The alarm, turned up

Shown upsetting pictures after a night without sleep, people's amygdalas reacted about 60% more strongly than after a normal night.

The brakes, weakened

Normally the medial prefrontal cortex keeps the amygdala in check. Without sleep their connection weakens, and as the prefrontal cortex's activity drops, anxiety rises. Deep sleep's slow waves do the reverse, calming it again.

The memory inbox

After a night without sleep, the hippocampus was far less active while people learned, and they remembered less afterwards. Sleep after learning matters too: during deep sleep the hippocampus replays the day's memories and hands them on to the cortex for keeping.

The night shift

In sleeping mice, the spaces between brain cells opened up by 60%, fluid flushed through faster, and waste, including the beta-amyloid linked to Alzheimer's, was cleared more quickly. Sleep may be when the brain cleans up.

Exams, all-nighters and school

Why sleep is part of studying, not time taken from it.

• 8–10 h what teens need: A night, for 13–17-year-olds; adults need 7 or more.
• < 1 in 3 high schoolers get 8 hours: On school nights, in the US.
• 8:30 the earliest schools should start: So students can get 8.5–9.5 hours.

Sleep works on both sides of learning. Before studying, it readies the hippocampus to take new material in: after a night without sleep, it can't do that as well. After studying, sleep is when those memories are replayed and stored. Cut either night short and you lose some of what you put in.

It isn't just willpower. The changes of puberty shift teenagers' sleep, and early school start times make it hard to get enough; paediatricians recommend schools start no earlier than 8:30. Too little sleep in teens is linked to poorer schoolwork, low mood and risky behaviour.

Sleeping better

Plain habits, but they're the ones the evidence backs.

What helps

• Go to bed and get up at the same time every day.
• Keep the bedroom quiet, relaxing and cool.
• Turn off screens at least 30 minutes before bed.
• No caffeine in the afternoon or evening; no big meals or alcohol before bed.
• Exercise regularly.

Don't drive tired

Too little sleep raises the risk of car crashes. After 17–19 hours awake, reactions can be as impaired as at a blood alcohol level of 0.05%, and worse the longer you stay up. Get a ride, or sleep first.

If you're getting enough hours in bed and still can't sleep, or still feel exhausted, that's worth talking to a doctor about.

Put it into practice

Four real situations. Choose what you'd do, and see why.

• It's 11 p.m. and your exam is at 9 a.m. You haven't finished the material. What gives you the best chance? Study a bit more, then sleep: sleep both stores what you've learned and keeps your hippocampus ready. During sleep the hippocampus replays and stores the day's learning, and without sleep it takes new material in poorly.
• You've been up since 7 a.m. It's 1 a.m., and a friend offers to let you drive their car home. How impaired are you, likely? Possibly as much as at a blood alcohol level of 0.05% — don't drive. After 17–19 hours awake, performance can match or be worse than at 0.05%.
• A friend sleeps five or six hours most nights and says she's adapted and feels fine. What does the research say? Her deficits are probably still building — feeling fine is exactly what people on short sleep report. On 6 hours or less, deficits kept building to the level of two nights without sleep, while sleepiness ratings levelled off.
• After a bad night's sleep, you snap at your roommate over something tiny and then feel awful about it. What's going on? Your amygdala is overreacting, and your prefrontal cortex has a weaker hold on it. After sleep loss the amygdala reacts about 60% more, with weaker prefrontal control.