Recovery: deep sleep, REM and HRV — what really matters

I often talk about load, progression, intensity. But the part almost nobody really wants to face is recovery — and sleep in particular, which is not “switching off for eight hours” but an active process, divided into phases that do completely different jobs.

Deep sleep: the repair phase

Slow-wave sleep (so-called “deep sleep”) is concentrated mostly in the first part of the night. This is when the body releases the largest share of growth hormone, restores glycogen stores, consolidates the immune system and repairs the tissues stressed by training. If this phase gets cut — going to bed late, alcohol, blue light until a minute before sleeping — you are not just “sleeping less”: you are shrinking the biological window in which the body does the most important part of its adaptation work.

REM sleep: the nervous-system phase

In the second part of the night REM sleep takes over, linked to memory consolidation — including motor memory, so the technical movements you are training — and to the regulation of the nervous and emotional systems. People who cut their sleep by setting an early alarm to train often sacrifice exactly this phase, which comes in the last hours. The result: the body recovers physically but the nervous system stays drained, and technical quality suffers even before strength does.

How much to sleep, really

The seven-to-nine-hour range applies to the general population. People who train seriously are almost always at the top end, and often beyond: load increases the need for repair, and that is the time available to do it. In studies where athletes were asked to add hours of sleep for a few weeks — not to sleep better, simply to sleep more — reaction times, accuracy and performance improved, without changing anything else in their programme.

That is why, when someone asks me what to add in order to improve, the first answer is often not an extra session. It is an extra hour of sleep.

The three things that ruin it

In practice, people who train and sleep badly almost always trip over one of these three.

Caffeine taken too late. Its half-life is around five to six hours, with marked individual differences: half of your five o’clock coffee is still circulating at midnight. You may fall asleep anyway and still get less deep sleep.

The evening drink. It helps you fall asleep faster, which is why it is mistaken for an aid. In reality it fragments the second part of the night and squeezes out precisely REM sleep, the phase the nervous system needs.

Hard sessions close to bedtime. It is not a universal rule — some people are not affected at all — but for many, the arousal and raised body temperature from hard work at nine in the evening are paid for when trying to fall asleep. If that is your only possible slot, training then is perfectly fine: you work on what comes after, with a cool-down routine and a few minutes of long-exhale breathing, which is the most direct way to bring arousal down (I explained it in more detail here).

HRV: a tool, not a verdict

Heart rate variability (HRV) measures how much the interval between one heartbeat and the next varies, and it is an indirect indicator of how balanced the autonomic nervous system is between activation (sympathetic) and recovery (parasympathetic). It is useful, but it has to be read properly:

• The trend counts, not the single reading. One isolated low value says little; a downward trend over several days says much more.
• It is individual. Comparing your HRV with someone else’s makes no sense: it only matters relative to your own baseline.
• It is affected by everything, not just training: alcohol, stress, illness, even a heavy meal the night before.

How I use it in programming

I do not make decisions from a single number. I look at sleep (duration and, when available, phases), HRV and perceived fatigue together, and use that data to decide whether an athlete is ready for a demanding session or needs to back off. Training load is only half the job: the other half is making sure the body really has the time and the conditions to turn that stimulus into improvement.

Recovery is not the break between two workouts. It is the part of training where adaptation actually happens.

Recommended listening

On this topic I recommend an episode of Andrew Huberman’s Huberman Lab podcast with Dr Andy Galpin, devoted precisely to how to maximise recovery to reach fitness and performance goals: Maximize Recovery to Achieve Fitness & Performance Goals — a long conversation but a genuinely good one, which goes much deeper into what I have summarised here.