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HRV (heart rate variability) is the variation in time between consecutive heartbeats, and it reflects how far your nervous system is leaning toward recovery rather than stress. Comparing your number to someone else's tells you almost nothing, because what counts as normal differs enormously between people. The useful signal is the trend against your own baseline over weeks, not any single morning's reading.

Normal training fatigue clears in a day or two, functional overreaching is a deliberate dip that pays off after a short recovery, and true overtraining syndrome keeps performance down for months. What separates them is not how bad one morning felt, it is whether resting heart rate, HRV, sleep quality, mood and performance at the same effort have all moved the same way for a week or more.

There is no single normal HRV range that applies to an individual: in a study of roughly 149,000 people, age and sex together explained only about a fifth of the differences in HRV between individuals. The only comparison that carries real information is your own rolling baseline, built from weeks of your own overnight data, telling you whether today is above, below or in range for you.

A rising resting heart rate trend means your body is working harder at rest than it usually has to, most often because of illness, alcohol, poor sleep, sustained stress, or training load that has outpaced recovery. One high morning is noise; a week sitting above your own baseline is a signal worth acting on.

A night of sleep runs in cycles of roughly 90 minutes through core (light), deep and REM sleep. Deep sleep concentrates in the first half of the night and handles physical repair, REM builds toward morning and handles memory and emotion. Chasing a specific deep sleep percentage is a poor goal, because deep sleep is protected when your night is short, which means a bad night can produce a better-looking percentage than a good one.