Of the five things your watch measures overnight, respiratory rate gets the least attention and has arguably the best signal-to-noise ratio. The reason is simple: it barely moves. HRV swings 20 percent night to night for perfectly ordinary reasons. Your breathing rate does not.
Why stability is the whole point
A metric is only useful as an alarm if its normal state is quiet. HRV is a noisy signal that requires a rolling average and a fortnight of context before a single reading means anything.
Respiratory rate is the opposite. Most people sit within a band about one breath per minute wide, night after night, for months. So a sustained move of one and a half breaths per minute, which sounds trivially small, is a large deviation in the context of how little this number normally moves.
What moves it
Infection. Often the earliest of the overnight signals to move, sometimes a day or two before symptoms. Typically accompanied by a rising heart rate and falling HRV, and it is the combination rather than any one that carries the information.
Alcohol. Raises it, along with everything else overnight. A dependable confounder.
Room temperature. Warmer rooms raise it modestly.
Altitude. The largest single effect on this list. Arriving somewhere meaningfully higher raises respiratory rate immediately and it stays raised for days.
Hard training. A small rise the night after a demanding session, which should resolve within a day or two.
Late meals and pregnancy both raise it, the latter substantially and persistently.
What it cannot tell you
Respiratory rate is not a screen for sleep apnoea. Apnoea is characterised by pauses in breathing and the oxygen desaturations that follow, not by average rate, and an average can look entirely normal across a night full of events. Some watches now have separate breathing disturbance features for exactly this reason. If you snore heavily, wake unrefreshed and feel sleepy during the day, that is a conversation with a doctor rather than a number to watch.
It also cannot distinguish causes. A rise from a cold, from altitude and from a warm bedroom look identical on the chart.
How to use it in practice
Treat it as a confirmer rather than a trigger. On its own, one elevated night is weak. Elevated respiratory rate plus elevated resting heart rate plus suppressed HRV, sustained across two or three nights, is one of the stronger patterns consumer hardware can produce, and is a reasonable prompt to take an easy day and go to bed early.
The important caveat, which we put actual numbers on in your resting heart rate knows first, is that even a genuinely accurate detector generates far more false alarms than true ones, because most nights are not the start of an illness.
The Vitals app takes a similar approach without the interpretation step, which we covered in the Apple Watch Vitals app explained.