Recovery

Sleep: the training variable you're ignoring.

By Luigi · 6 min read

People will spend £60 a month on supplements with barely any evidence behind them, then sleep five and a half hours a night and wonder why nothing's moving. If I could only fix one thing outside the gym for most clients, it wouldn't be their protein. It would be their sleep.

A note before we start, because I'd rather be honest than dramatic: sleep science is often oversold. Some of the scariest claims you'll see online come from small studies, short timeframes, or animal work stretched to breaking point. So below is what the research actually shows — including where it's thin — and why it still adds up to sleep being one of the highest-leverage things you control.

What short sleep does to your training

A systematic review of sleep and athletic performance found that sleep loss reliably degrades performance — but not evenly^[1]. Sustained, repeated efforts and skill suffer most; a single maximal lift holds up surprisingly well. A more recent meta-analysis sharpened that picture: the later in the day and the longer you've been awake, the bigger the drop, and anything under about six hours is where performance measurably falls apart^[2]. In practice: you can probably survive one bad night and still deadlift fine at 8am. A whole week of six-hour nights, training at 7pm? That's where your sessions quietly get worse.

Muscle: the part most people miss

One night of total sleep deprivation has been shown to reduce muscle protein synthesis — the process that actually builds muscle — by around 18%, alongside a rise in cortisol and a drop in testosterone^[3]. That's a single small study on total deprivation, so hold it loosely — but it points the same direction as everything else in this area: you don't grow in the gym, you grow while you recover, and sleep is when most of that happens.

Fat loss: the study that should change your behaviour

This is my favourite study in this whole area, because it's a proper controlled experiment rather than a survey. Dieters were put on identical calorie deficits — one group sleeping 8.5 hours, the other 5.5. Both groups lost the same weight. But the short-sleep group lost 55% less of it as fat, and more of it as muscle^[4]. Same diet, same deficit, different sleep — and one group traded fat loss for muscle loss. If you're dieting hard on short sleep, you're doing the hard part and skipping the part that makes it work.

Mechanistically that fits what we know: even a couple of short nights shifts hunger hormones — ghrelin up, leptin down — and measurably increases appetite, especially for calorie-dense food^[5]. Short sleep doesn't just blunt results; it makes the diet harder to stick to at the same time.

Injury: worth knowing, with a caveat

In adolescent athletes, sleeping under eight hours was associated with 1.7× the injury rate^[6]. Caveat, because I said I'd give you them: that's observational data in teenagers, not adults in a gym — it shows an association, not proof of cause. But combined with what sleep loss does to coordination and effort, I wouldn't bet against it applying to the rest of us.

What I actually tell clients

• Aim for 7–9 hours. That's the adult guideline^[7], and most people I coach are under it, not over it.
• Fix the schedule before the gadgets. A consistent bedtime does more than any tracker, tape, or supplement. Boring, free, effective.
• One bad night changes nothing. Don't skip the session because you slept badly — train, maybe expect a little less, and protect tonight instead.
• If you're dieting, sleep is part of the diet. See the 55% study above. I programme around your real schedule — that's why I start sessions from 5am — but I'll also tell you when the schedule itself is the problem.