Pulled an All-Nighter? A 20-Minute Workout Protected Memory as Well as a Nap

You were up all night. Maybe it was a baby, a deadline, a night shift or a red-eye flight.

Now you have to learn something important today. A new process at work, a set of names, a briefing you can't afford to forget.

The usual advice is to take a nap. But what if you can't?

A new study in PNAS offers a surprising alternative. After 30 hours awake, a 20-minute workout on an exercise bike protected memory about as well as a 90-minute nap.

And brain-wave recordings suggest the two work in completely different ways.

Why sleep loss hurts memory

Sleep isn't only about feeling rested. It also gets your brain ready to learn.

When you're short on sleep, the brain gets worse at "encoding". That's the first step of making a memory, where new information gets written in.

So you can sit through a meeting after a bad night and later find that much of it simply didn't stick.

That matters for everyone. But it matters most for nurses, doctors, pilots, drivers and shift workers, who often have to learn and remember while exhausted.

The experiment: 30 hours awake

Who took part

Researchers at McGill University and a rehabilitation research centre near Montreal recruited 54 healthy adults aged 18 to 35. None had sleep disorders or worked night shifts.

One person was later excluded, leaving 53 in the main memory analysis.

What they did

Everyone woke up at home as usual. They came to the lab at 9pm and stayed awake all night under supervision.

After 30 hours awake in total, each person was randomly put into one of three groups:

The exercise was brisk but not brutal. Think of a steady cycle where talking gets hard, rather than an all-out sprint.

Everyone then waited 30 minutes. This gave the nappers time to shake off post-nap grogginess before the test.

The memory test

Next, everyone looked at 150 everyday photos and rated each one. They wore a cap with 64 sensors that recorded their brain waves (EEG).

They weren't told they'd be tested later.

Three days later, after catching up on sleep, they came back for a surprise quiz. They saw the 150 old photos mixed with 75 new ones and had to say which they had seen before.

The results: both worked, and about equally

Here's how many of the old photos each group correctly recognised.

Both the exercise and nap groups beat the control group. On average, that was a 22% boost.

The exercise and nap groups weren't meaningfully different from each other.

In statistical terms, the effects were large. But the exercise result was right on the edge of the usual cut-off for significance, so it's a promising result rather than a settled one.

The fatigue twist

You might assume the exercise group would feel wrecked afterwards. They didn't.

The nap group, unsurprisingly, felt much less tired after sleeping.

But the exercise group felt no more tired than the people who just sat on the bike. Twenty minutes of cycling didn't add to their exhaustion.

Even so, they remembered more than the control group, despite feeling just as tired.

Same result, different brain paths

This is the part I find most fascinating.

The brain-wave recordings suggest exercise and naps protected memory in quite different ways.

The nap reset the brain

After a long time awake, the brain shows more slow waves while you're still conscious. Researchers see this as a sign of "sleep pressure", the built-up need to sleep.

The nap lowered these slow waves in the frontal and task-related parts of the brain. In other words, it partly cleared the backlog.

For nappers, how well they remembered was linked mostly to this calmer, more refreshed brain state before each photo appeared.

Exercise sharpened the moment of learning

Exercise lowered some signs of sleep pressure too, but by less than the nap.

What it seemed to do instead was help the brain process each photo more efficiently as it appeared.

For the exercise group, memory was linked to brain signals tied to paying attention and storing that specific image.

The control group was working harder for less

In the control group, none of these brain signals predicted memory.

Instead, the signals tracked how tired people felt. The authors think their brains were straining to compensate, without much success.

The researchers describe this as "degeneracy", which is less alarming than it sounds. It means the brain can reach the same result through different routes.

What earlier research found

This study builds on two separate lines of evidence: sleep loss hurts memory, and naps and exercise can both boost thinking.

Sleep loss blocks new memories

A landmark 2007 study in Nature Neuroscience found that one night without sleep reduced activity in the hippocampus while people learned. The hippocampus is a key memory hub in the brain.

As a result, people remembered less later on.

Fitter people may cope better

The same Montreal-area research group had tested whether fitness protects memory from sleep loss in 29 young adults.

After 30 hours awake, the sleep-deprived group was clearly worse at recognising photos they had seen. The new study notes that higher fitness was linked to better memory in that situation.

That hinted exercise might help. The new study tested it directly.

Naps help, modestly

A 2021 review pooled 11 studies with 381 working-age adults that tested short daytime naps.

Napping improved thinking overall, with the clearest gains in alertness. The benefit mostly lasted up to 2 hours after the nap.

The effects were small, but consistent.

Exercise helps memory, depending on timing

A 2019 review of 25 studies on single workouts and memory found that timing mattered.

Exercise just before learning gave only a small boost that fell short of statistical significance. Exercise shortly after learning helped more.

Earlier exercise studies were mixed

The new paper also reviews earlier sleep-loss studies. Some found that short bouts of exercise helped attention after a night awake.

Others found no benefit, or even harm, especially when people exercised repeatedly throughout the night.

The authors think the dose matters. One solid 20-minute session seems to help, while many bouts across a sleepless night may just add stress.

How it fits together

Put together, earlier research told us sleep loss damages memory, and that naps and exercise can each give the brain a lift.

What's new is the head-to-head comparison after real sleep loss. It suggests exercise may be a serious alternative to napping, not just a weaker substitute.

How much should you trust this?

Promising. This is a well-designed randomised experiment, but it's small and done in a lab.

What makes it convincing

• People were randomly assigned to groups, and the groups were similar in age, fitness and sleep habits.
• The all-night wakefulness was supervised in the lab, not self-reported.
• Memory was tested 3 days later, after everyone had caught up on sleep, so it measured what really stuck.
• Published in PNAS, a major peer-reviewed journal. The authors declared no competing interests and shared their data openly.

What makes me cautious

• Only 53 people were analysed, with 17 or 18 per group. Small studies can overestimate effects.
• The exercise result was only just statistically significant.
• The groups differed in how many old photos they recognised, but not in a stricter score that also counts false alarms. The exercise group had slightly more false alarms, though that difference wasn't significant.
• Everyone was young and healthy. We don't know if the same works for older adults or people with health conditions.
• One night of lab sleep loss isn't the same as months of chronic poor sleep or rotating shifts.
• There was no "nap-length" control, because keeping that group awake 90 minutes longer would have made the comparison unfair.

What this means for you

The best fix for sleep loss is still sleep. Nothing in this study changes that.

But if you've had a rough night and need to learn something important, here's what the evidence suggests:

• Nap if you can. A longer nap, followed by time to shake off the grogginess, protected memory in this study.
• If you can't nap, move. A brisk 20-minute session, where talking gets hard, was about as helpful for memory here.
• Give yourself a buffer. In the study, learning started 30 minutes after the nap or workout.
• Don't overdo it. Earlier research suggests repeated hard bouts through a sleepless night may backfire.
• Stay safe. Don't exercise hard if you feel unwell, dizzy or have a heart condition, and never drive drowsy.

If poor sleep is a regular problem, the NHS explains why lack of sleep is bad for your health and how to get help.

In this TED talk, sleep scientist Matt Walker explains why sleep before learning matters for memory:

What we still don't know

1. Does it work in the real world? The authors want to test it with healthcare, mining, transport and shift workers.
2. Would a shorter nap work too? Many people can't take 90 minutes off.
3. How long does the benefit last? Learning here started 30 minutes after the workout.
4. What's the best dose of exercise? Shorter, gentler or longer sessions haven't been compared.
5. Does it help other skills? This study only tested memory for pictures, not attention, reaction time or decision-making.
6. Could exercise plus a nap work even better? Nobody has tested the combination yet.

My take: a useful backup plan

What I like about this study is how practical it is. Most people can't nap at work, but many could find 20 minutes to move.

I'm cautious about the size of the effect, because the groups were small. I'd want to see it repeated in real workplaces before anyone rewrites the rules for night shifts.

Still, it's a welcome finding. After a bad night, "go for a brisk ride" may be more than a pep talk.

It may genuinely help your brain hold on to what you learn.