Heart rate variability is the most over-interpreted number in consumer health. It is genuinely useful, and it is also noisy, highly individual and easily moved by things that have nothing to do with your training. Here are the causes ranked by how often each one is actually the answer.
1. Alcohol, the night before
The single most reliable HRV suppressor available to a civilian. Two drinks will show up in that night’s reading and frequently in the next night’s too. The effect is dose-dependent and outlasts the alcohol clearing your bloodstream, because what is being disrupted is autonomic activity during sleep rather than the alcohol itself.
If you drank and your HRV dropped, stop looking. That is the cause. Full detail in how long alcohol affects your HRV and sleep.
2. A late or heavy meal
Digestion is sympathetic activity, and going to sleep with a large meal still being processed keeps that system busier than it should be overnight. Eating close to bedtime is one of the most underrated causes of an unexplained low reading, and one of the easiest to test on yourself.
3. Short or broken sleep
Both duration and fragmentation matter. A five-hour night depresses HRV even without any other insult, and a night broken into pieces does so even at normal total duration. Since most watches sample HRV predominantly during sleep, a short night also means fewer samples and a noisier estimate, so the reading is both genuinely lower and less reliable.
4. Hard training in the last 24 to 72 hours
This is the one people jump to first and it belongs fourth. A hard session should suppress HRV. That is the adaptive response doing its job, not a warning. The pattern that matters is failure to rebound: if HRV is still down four days later with no further hard work, something else is going on.
5. Illness, often before you feel it
HRV frequently drops a day or two before symptoms, usually alongside a rising resting heart rate and respiratory rate. This is real, and it is also where most people over-read their data: the great majority of these patterns turn out to be alcohol, travel or a bad night rather than infection.
We put actual numbers on how often that inference is wrong in your resting heart rate knows first, and the honest answer is that a genuinely accurate detector still generates mostly false alarms.
6 to 9, the rest of the list
Dehydration. Lower plasma volume, higher heart rate, lower HRV. Common after a long flight or a hot day.
Psychological stress. A genuinely stressful day registers overnight. HRV does not distinguish between kinds of load.
Travel and altitude. Time zone shifts suppress HRV for several days. Altitude does so immediately and substantially.
A loose watch. The dull one. A band that moves during the night produces poor optical contact, fewer usable samples and a reading that reflects the strap rather than your nervous system. If a low number appears with no plausible cause, check the fit before theorising.
When a low HRV is actually worth acting on
The useful signal is almost never one morning. It is the combination of:
HRV below your own baseline for several consecutive days, a resting heart rate that has drifted upward over the same period, and a subjective sense that normal training feels harder than it should. Any one of those alone is weak evidence. All three together is a good reason to take an easy week.
If that pattern persists for more than a fortnight with no obvious explanation, it stops being a training question. See a doctor rather than an app.
If you want to raise the baseline rather than explain a single night, the training and habit changes that actually work are in how to improve your HRV. If your HRV chart is empty rather than low, see Apple Watch HRV not showing.