Why a short night wrecks your next workout

After a short night you can still train, but each unit of effort buys you less speed or power than usual.

In a 2009 study, 11 men stayed awake for 30 hours and then ran as far as they could in 30 minutes on a treadmill.² They covered about 6,040 meters, compared with about 6,220 meters after a normal night of sleep. That's 190 meters short, a drop of roughly 3 percent in a single half hour.

The interesting part is what didn't change. Effort ratings and heart rates were about the same on both days.² The runners felt they were working just as hard, while their legs quietly settled on a slower pace.

A 2025 review of 45 studies points the same way from the other side.³ After sleep loss, people rated a fixed amount of work as harder than they did when rested.

Bottom line

Sleep loss lowers what you get from a given effort. Expect a slower pace on the same feeling, not a different feeling.

The average cost is a few percent, and it shows up in almost every kind of exercise. Sprints, lifting, intervals, endurance work and skill work all got worse in the 2022 review.¹

How the sleep went missing made a difference. Staying up all night and waking earlier than usual both hurt the next session consistently.¹ Going to bed late and waking at the usual time didn't produce a clear effect.

The time of day mattered as well. Morning sessions were mostly unaffected, but afternoon and evening sessions suffered.¹ Every extra hour awake before the session cost about 0.4 percent more, so time spent awake seems to matter alongside the sleep you lost.

Nearly 9 in 10 of the 959 people in those studies were men.¹ The direction of the effect is clear, even if its exact size for you isn't.

Bottom line

Missing sleep costs every kind of training a few percent. Waking early and then training late is the worst combination.

People who sleep less tend to get hurt more often, but nobody has shown that one bad night causes an injury.

The number you'll see quoted most often comes from a 2014 survey of 112 athletes aged 12 to 18.⁴ Those who usually slept under 8 hours were 1.7 times as likely to have been injured as those who slept longer.

That study asked about usual sleep rather than single nights, and it looked only at teenagers. It can't separate sleep from everything else that comes with a crowded schedule. We found no trial testing whether one short night raises injury risk in adults.

Bottom line

Treat weeks of short nights as an injury risk. Treat one short night as a reason to go easier, not as a reason to worry.

After a short night you have three sensible options. You can move the hard session earlier, make it easier, or get a little sleep before it. Each one is supported by different evidence.

Moving it earlier works because morning sessions lost the least in the 2022 review.¹ If your day allows it, do the intervals at 7 a.m. instead of saving them for 6 p.m., when you've been awake for twelve hours.

Making it easier works because your sense of effort stays honest even when your pace drops.² Run by feel and let the watch show a slower number without chasing it. If your plan calls for a hard workout, swap it for an easy run and move the hard work to the next good morning.

A nap is the third option for a tired day. A 2021 review of 18 studies found that a daytime nap improved physical performance in most of them, including after a night of restricted sleep.⁵ The longest naps, about 90 minutes, produced the biggest benefit.

Bottom line

After a short night, train early, train easy, or nap first, and save the hardest work for a rested day.

Most adults need 7 to 9 hours of sleep a night, according to a 2015 expert panel convened by the US National Sleep Foundation.⁶

Extra sleep seems to help in the other direction too. In a 2011 study, 11 college basketball players spent at least 10 hours in bed each night for 5 to 7 weeks.⁷ They ended up sleeping almost 2 hours more per night. Their sprint time dropped from 16.2 to 15.5 seconds, and they made 9 percent more of their free throws.

It was one small study, and each player was compared only with his own earlier weeks, so treat the size of the gain loosely.⁷ It also required 10 hours in bed every night, which few adults with jobs and families will manage.

Bottom line

One short night costs a little. Weeks of short nights cost more, and longer sleep may win some of it back.

1. 01

   One short night lowers your output by a few percent, while the effort feels the same or harder.

2. 02

   Staying up all night or waking early does the most harm, and late sessions suffer more than early ones.

3. 03

   The link between short sleep and injury comes from surveys of teenagers, not from trials.

4. 04

   After a short night, train early, train easy, or nap first.

5. 05

   Most adults need 7 to 9 hours of sleep a night.