The science of sleep

Sleep stages, circadian rhythm, melatonin and jet lag — and what they do to recovery.

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• How Does Last Night's Sleep Turn Into a 1–5 Condition Score for Today's Workout?

  How your sleep stages, cycles, and light exposure shape a 1–5 morning condition score—and how to use it to adjust today's workout load.

• How does melatonin timing shape your sleep and next-day training?

  Melatonin is the hormone your brain releases at night to signal sleep timing. Light, travel and late screens shift it, and that shows up in recovery.

• Is deep sleep the part of the night that matters most before a hard workout?

  Slow-wave sleep, often called deep sleep, clusters early in the night. Use a short deep-sleep night as a reason to check other signals, not to panic.

• Should a night with little REM sleep change how hard you train today?

  REM sleep is the dream-rich phase that returns several times a night. A short REM night is a cue to adjust, not a verdict on your fitness.

• Should you still train after a night of bad sleep, or take the day off?

  One short night rarely ruins a session, but sleep loss adds up and dulls attention. Lower the intensity, protect technique, and sleep more tonight.

• The 3 Rules That Define a True Circadian Rhythm — And Why Your Recovery Data Follows Them

  A rhythm only counts as circadian if it meets three specific tests. Here's what those rules are and why they explain your sleep, HRV, and jet lag.

• Training after a long-haul flight: how jet lag changes what your body can handle

  Jet lag is a mismatch between your body clock and local time. Expect poorer sleep and focus for days, and plan lighter sessions while you adjust.

• Which sleep hygiene habits actually help you recover for tomorrow's training?

  Sleep hygiene is a set of everyday habits around the bedroom and evening. Steady routines make your overnight recovery data easier to read and act on.