When someone isn’t progressing in the gym, the usual reaction is to look at the training programme.

Do we need more volume? More weight? More protein? More calories?

Those are all sensible questions. But there is another variable I look at with clients before we start adding more work:

How well are you actually sleeping?

Not because sleep is some secret anabolic hack. And certainly not because eight hours in bed can compensate for poor training or an inadequate diet.

It can’t.

But resistance training doesn’t build muscle in isolation. Training provides the stimulus. Your body then has to recover from that stimulus and adapt to it.

Sleep appears to influence both sides of that equation.

And when sleep becomes chronically inadequate, you may be training just as hard while getting less back from the work you’re putting in.

What actually happens when you lose sleep?

One of the most useful studies in this area came from Lamon and colleagues in 2021.

Thirteen healthy young adults completed both a normal-sleep condition and a night of complete sleep deprivation. The researchers then used stable-isotope tracers and muscle biopsies to directly measure muscle protein synthesis.

After the sleepless night, post-meal muscle protein synthesis was approximately 18% lower.

Cortisol was around 21% higher, while circulating testosterone was approximately 24% lower. [1]

That’s important because muscle protein synthesis is one of the fundamental processes through which we repair and remodel skeletal muscle.

But there is an important caveat.

This was one night of complete sleep deprivation.

That isn’t the same thing as someone regularly sleeping six rather than eight hours. So I wouldn’t tell a client that one bad night has suddenly reduced their muscle growth by 18%.

It hasn’t.

What the experiment tells us is that severe sleep loss can directly alter the anabolic environment of skeletal muscle.

And chronic restriction gives us another piece of the picture.

Several short nights may matter more than one bad night

Saner and colleagues studied healthy young men undergoing five nights with only four hours of time in bed per night.

Rates of myofibrillar protein synthesis were lower during sleep restriction.

Interestingly, a group performing high-intensity interval exercise during the restriction period maintained protein-synthesis rates closer to the normal-sleep condition. [2]

That doesn’t mean you can out-train poor sleep.

The exercise intervention was HIIT rather than a normal hypertrophy programme, the study was short, and muscle protein synthesis is not the same thing as measuring actual muscle growth over several months.

But it reinforces an important coaching principle:

Exercise provides a protective stimulus, but recovery still matters.

You don’t have to stop training because you’ve had a rough week of sleep.

You should, however, recognise that five nights of four-hour sleep is a very different recovery environment from five nights of seven or eight hours.

Poor sleep can also reduce the quality of the training itself

Muscle growth doesn’t just depend on what happens after training.

Sleep can affect how much productive training you are capable of doing in the first place.

A large 2022 systematic review and meta-analysis examined 69 studies and 227 performance outcomes. Across the data, acute sleep loss negatively affected exercise performance, including strength, strength-endurance, speed, power and endurance.

The average reduction across all performance measures was around 7.6%, although results varied considerably between studies and protocols. [3]

More recent evidence tells a similar story.

A 2025 systematic review examining sleep deprivation and muscular strength found that not every measure deteriorates after sleep loss, but reductions in strength, power, muscular endurance and neuromuscular function occur often enough to matter for people involved in strength-based exercise. [4]

This is where sleep becomes particularly relevant from a coaching perspective.

Imagine two versions of the same athlete performing the same programme.

One arrives recovered and performs:

100 kg × 10
100 kg × 10
100 kg × 9

The sleep-deprived version performs:

100 kg × 9
100 kg × 8
100 kg × 7

Neither workout is necessarily “bad”.

But repeat that difference across dozens of sessions and the well-recovered athlete may accumulate more high-quality repetitions, maintain higher bar velocities and progress loads more consistently.

That creates a potential pathway from poor sleep to poorer long-term adaptation without sleep ever having to directly “switch off” muscle growth.

We now have useful data in trained women as well

Earlier sleep research was heavily male-dominated, which was a significant limitation.

More recent studies have started addressing this.

Knowles and colleagues studied resistance-trained women during nine nights of either normal sleep or restricted sleep, with the restricted condition allowing five hours in bed per night.

Training volume was surprisingly well maintained.

But training quality deteriorated.

Lower-body concentric velocity was reduced by as much as approximately 15% during some sets. Velocity loss within sets increased, session RPE was higher, training distress increased substantially and salivary cortisol exposure increased. [5]

That’s an interesting finding for coaches because a client can technically “complete the programme” while the internal cost of completing it is becoming much higher.

On paper:

4 × 8 squats completed.

Programme followed.

Job done.

Physiologically, however, those sessions may not be equivalent.

The same research group subsequently showed that nine nights of sleep restriction also altered how skeletal-muscle genes responded to repeated resistance exercise in trained women. Thousands of transcripts responded to training, but the pattern of response differed substantially between normal and restricted sleep. [6]

That doesn’t prove those women would grow less muscle over six months.

But it gives us another reason to avoid viewing sleep simply as whether someone feels tired the following morning.

Does poor sleep actually stop hypertrophy?

This is where we need to be careful.

The mechanistic case is strong.

The performance case is strong.

The evidence that extreme sleep restriction alters muscle protein synthesis is fairly convincing.

But direct long-term trials measuring actual muscle hypertrophy under different sleep conditions are still limited.

And not every study finds that modestly shorter sleep prevents resistance-training adaptations.

A 2024 study compared adults who habitually slept less with adults obtaining closer to recommended sleep durations during a short resistance-training intervention.

The shorter-sleep group averaged roughly 5 hours 47 minutes of night-time sleep, compared with around 7 hours 16 minutes in the adequate-sleep group.

After 16 resistance-training sessions, both groups improved measures including repetition performance and arm muscle area, and the researchers did not find evidence that the modest difference in habitual sleep prevented the positive adaptations to resistance training. [7]

This study was small and relatively short, so it doesn’t close the question.

But it’s an important counterpoint.

Sleeping six hours does not make muscle growth impossible.

Your physiology isn’t that fragile.

What is more reasonable to say is that consistently inadequate sleep can make the overall environment for maximising hypertrophy less favourable—particularly when sleep becomes severely restricted or is combined with high training stress, dieting or other recovery demands.

Sleep becomes particularly important when you’re dieting

For anyone trying to lose fat while preserving muscle, the sleep research becomes even more interesting.

One of the classic experiments comes from Nedeltcheva and colleagues.

Ten adults with overweight completed two 14-day periods of calorie restriction.

Calories were controlled.

The major difference was sleep opportunity:

8.5 hours versus 5.5 hours per night.

Total weight loss was fairly similar.

But where that weight came from was very different.

With the longer sleep opportunity, participants lost approximately:

1.4 kg of fat
1.5 kg of fat-free mass

With the shorter sleep opportunity:

0.6 kg of fat
2.4 kg of fat-free mass

In other words, reducing sleep opportunity decreased the amount of weight lost as fat by approximately 55% and increased fat-free-mass loss by roughly 60%. [8]

It was only ten people, so we shouldn’t build an entire fat-loss philosophy around one experiment.

Fortunately, another longer study points in the same general direction.

Wang and colleagues studied adults undergoing eight weeks of calorie restriction. One group also had approximately one hour of sleep restriction on five nights per week.

Both groups lost weight, but the adequately sleeping group lost a significantly greater proportion of that weight from fat. Weekend catch-up sleep did not appear to completely compensate for the restriction during the week. [9]

From a body-composition coaching perspective, that’s significant.

When calories are already low, recovery resources are reduced.

Adding chronic sleep restriction on top of that may make preserving lean tissue and producing good training sessions unnecessarily difficult.

What about testosterone, growth hormone and cortisol?

This part of the sleep discussion often becomes oversimplified.

You’ll sometimes hear:

“You grow while you’re sleeping because growth hormone is released at night.”

There’s some truth buried in that statement, but human muscle growth is considerably more complicated.

Deep slow-wave sleep is associated with substantial growth-hormone secretion, and sleep also influences normal daily patterns of testosterone and cortisol.

Acute severe sleep deprivation can reduce testosterone and increase cortisol, as demonstrated in the Lamon experiment. [1]

But it would be misleading to claim that slightly reducing a nocturnal growth-hormone pulse directly causes a predictable reduction in hypertrophy.

Skeletal-muscle adaptation is regulated by an entire network involving mechanical tension, intracellular signalling, muscle protein turnover, nutrient availability, training status and numerous endocrine and local growth factors.

The hormonal response is therefore better viewed as one part of the recovery environment, not the master switch controlling muscle growth.

For coaches, that distinction matters.

I wouldn’t try to “optimise growth hormone”.

I’d try to make sure the athlete is training progressively, eating enough protein and energy, and sleeping sufficiently for that workload.

Sleep also affects how easy your nutrition plan is to follow

There is another practical issue that rarely gets discussed in hypertrophy articles.

A tired client doesn’t only recover differently.

They often behave differently.

Experimental sleep restriction has been shown to increase energy intake and can promote weight gain when food is freely available.

In one controlled 21-day crossover study, restricting sleep to four hours per night increased calorie intake without a compensatory rise in energy expenditure. Participants gained more weight and accumulated more abdominal fat during the sleep-restricted condition. [10]

That’s relevant whether you’re gaining or cutting.

During a fat-loss phase, increased hunger makes adherence much much harder.

During a gaining phase, poor appetite control can turn a controlled surplus into a much larger one.

Sleep therefore influences body composition through more than muscle physiology alone.

Can sleeping more actually improve performance?

Possibly—and this may be the more useful question for many athletes.

Mah and colleagues famously asked Stanford basketball players to increase their sleep opportunity to approximately ten hours in bed per night for several weeks.

The athletes increased their actual sleep by around 110 minutes per night.

Sprint performance improved, shooting accuracy improved, reaction time improved and daytime sleepiness decreased. [11]

The study was small and wasn’t designed to measure hypertrophy.

But subsequent research broadly supports the idea that increasing sleep opportunity can benefit athletic performance, particularly when athletes begin from an inadequate baseline.

A 2023 systematic review of sleep interventions in athletes found that sleep extension and napping were among the most consistently useful strategies for improving subsequent performance, although the overall quality and quantity of evidence remain limited. [12]

So if you’re already doing the obvious things well—training consistently, eating enough protein and managing your calories—sleep extension may be one of the few remaining recovery interventions with a meaningful upside and virtually no financial cost.

So how much sleep should someone trying to build muscle get?

For general adult health, major sleep organisations recommend regularly obtaining at least seven hours, with approximately 7–9 hours being an appropriate range for most adults. [13]

Athletes are slightly more complicated.

An expert consensus on sleep in athletes specifically cautions against treating one sleep number as universally optimal. Training load, age, individual sleep requirement, competition schedule, previous sleep debt and napping all influence how much sleep someone may need. [14]

For coaching purposes, I use the following framework:

Around 7 hours should be viewed as a lower boundary for most people rather than an aspirational target.

For someone training hard four or five days per week, 7.5–9 hours of actual sleep is a very reasonable target range.

Some athletes will perform best toward the top of that range.

Others will genuinely feel excellent closer to seven-and-a-half.

The important thing is not chasing an arbitrary number.

Look at the athlete.

Are they waking reasonably refreshed?

Is training performance progressing?

Are they recovering between sessions?

Is motivation stable?

Is resting fatigue creeping upward?

Are they relying on increasingly large amounts of caffeine?

Are they sleeping considerably longer whenever given the opportunity?

Those observations often tell you more than an impressive-looking sleep score from a wearable.

What I would do with a client who isn’t sleeping enough

I wouldn’t completely rebuild their programme after three bad nights.

I’d first find out why sleep is short.

If someone has eight hours available but only sleeps six because they’re scrolling on their phone until 1 a.m., that’s primarily a behavioural problem.

If they have a newborn baby, rotating shifts or genuinely unavoidable work demands, the coaching response needs to be different.

In that situation I may temporarily reduce unnecessary training volume, avoid piling failure work onto every exercise and concentrate the programme around the sets most likely to produce an adaptation.

That’s one of the biggest mistakes I see with recovery.

Someone feels under-recovered, so they decide they need to train harder.

Often they don’t.

They need to make the training they can recover from count.