You can train perfectly and eat right, but if you do not sleep, your body does not recover — it accumulates fatigue. Sleep is not “rest”. It is the time when the body repairs muscle, files memory away, balances hormones and charges the nervous system for tomorrow. Everything you see in tri — recovery, readiness for load, an even heart rate — starts here.

What we measure at night

While you sleep, the band quietly takes several signals:

  • How long you slept — not how long you lay there, but actual sleep time.
  • Stages — how much deep, light and REM sleep there was.
  • Heart rate and its variability (HRV) — at night they show most honestly how the body is recovering.
  • Breathing and skin temperature — an even baseline at night means all is calm; sharp deviations are the first hint that you are getting ill or overloaded.

None of these numbers is a verdict on its own. What matters is the whole picture — and that is what we assemble.

The stages, and what each is for

Sleep runs in cycles of about ninety minutes, and in every cycle you pass through the stages. On the night chart you can see it as steps: the line dips into deep sleep and climbs back up to REM.

  • Deep sleep — physical repair. This is where muscle is rebuilt, immunity strengthens, growth hormone is released. There is more of it in the first half of the night. Usually it makes 13–23% of the night.
  • REM sleep — repair for your head. The brain processes emotion and locks in what you learned during the day. There is more of it towards morning — so if you cut the last hours short, it is memory and mood that suffer. The norm is about 20–25% of the night.
  • Light sleep — the link between stages. There is always a lot of it — about half the night, and that is normal.
  • Awake — brief wake-ups between cycles. Surfacing for a minute or two three to five times a night is normal: you usually do not even remember it. What should catch your attention is long wake-ups adding up to more than a tenth of the night.

Do not chase “maximum deep sleep”. Healthy sleep is when all the stages run their course, night after night.

Sleep debt

If for several nights in a row you sleep less than your body needs, the difference accumulates — that is sleep debt. One long night will not repay it in full: recovery is gradual. tri shows the debt so you see not a single night but a trend — and understand where the sluggishness came from, even if last night seemed fine.

How the sleep score is built

We do not count points for the sake of points. The score answers one question: how much did this night restore you? It takes in:

  • whether there was enough time for you specifically (not an abstract “eight hours”);
  • whether all the stages got enough room;
  • how evenly you slept, without frequent waking;
  • whether you went to bed at roughly your usual time.

So two seven-hour nights can score differently: broken sleep restores you worse than solid sleep.

How to sleep better

A few things that genuinely work:

  1. Go to bed and get up at roughly the same time. Regularity matters more than duration — the body needs rhythm.
  2. No bright screen in the last hour. Light tells the brain “it is still day” and pushes falling asleep later.
  3. A cool bedroom. To fall asleep the body needs to cool slightly; in heat, deep sleep breaks up.
  4. Coffee before lunch. It stays in the blood for six to eight hours and quietly steals deep sleep.
  5. Alcohol puts you to sleep but does not restore you. After it HRV falls and REM sleep all but disappears — the band will show it.

Start with one point, not all at once. Look a week later at how the stages and the score changed — your body answers in numbers.

Sources

  • Sleep debt accumulates and is not repaid in one night — Van Dongen H. et al., Sleep, 2003. read
  • Caffeine even 6 hours before bed measurably worsens sleep — Drake C. et al., J. Clin. Sleep Med., 2013. read
  • Alcohol suppresses REM sleep and breaks up the second half of the night — Ebrahim I. et al., Alcohol Clin. Exp. Res., 2013. read
  • A bright screen in the evening shifts the body clock and delays falling asleep — Chang A. et al., PNAS, 2015. read
  • Sleep regularity matters more for health than sleep duration — Windred D. et al., Sleep, 2024. read

tri helps you understand your sleep but does not replace a doctor. If your sleep is consistently poor, if there is snoring with pauses in breathing, or daytime sleepiness — it is worth seeing a specialist.