Your overnight heart rate is the cleanest cardiovascular measurement your watch takes all day. You are lying still, unstimulated, with no caffeine, conversation or stairs confounding it. That is exactly why it is worth understanding, and why a change in it is meaningful in a way that a change in your daytime heart rate is not.
What normal looks like
For most healthy adults the overnight minimum sits somewhere between the high 40s and the low 60s, generally several beats below the resting figure your app reports for the day. Well-trained endurance athletes are often considerably lower, in the 30s and 40s, which is a normal adaptation rather than a problem.
That range is wide, and deliberately so. Genetics, age, sex, fitness and medication all shift it. A rate of 52 tells you nothing on its own. A rate of 52 when you have averaged 44 for the past month tells you a great deal.
The shape of the curve
This is the part most people never look at, and it carries more information than the single minimum.
A typical healthy night falls fairly quickly after sleep onset, reaches its lowest point in the first half of the night, and then drifts gradually upward toward morning as the body prepares to wake.
The pattern worth noticing is a curve that stays elevated for hours and only reaches its minimum shortly before you wake. That shape is characteristic of alcohol and of eating late, and it is why a night can produce a normal-looking average while having been physiologically poor.
What moves it, in order of likelihood
Alcohol. A reliable five to ten beat rise, sometimes more, and it commonly persists into the following night. The most frequent explanation for an unexpected overnight rise by a wide margin.
A late or large meal. Digestion keeps the sympathetic system engaged. Eating within two hours of bed reliably lifts the whole curve.
Hard training in the last 48 hours. Expected and appropriate. The pattern to watch is one that does not return to baseline after easy days.
A warm room. Thermoregulation costs cardiac output. Cooler rooms produce lower overnight rates in most people.
Dehydration. Lower plasma volume means a higher rate to maintain output. Common after flights and hot days.
Infection. Often a rise of several beats a day or two before you feel anything. Genuine, and also the explanation people reach for far too quickly. How often that inference is actually wrong is the subject of your resting heart rate knows first.
When to be concerned
A sustained rise of several beats over a week or more, without an obvious cause and alongside a suppressed HRV, is the pattern worth acting on. Usually it means back off training and sleep more, which is covered in the signs of overtraining.
Separately, an unusually low overnight rate accompanied by dizziness, fainting or breathlessness is a reason to see a doctor rather than to feel pleased with your fitness. Nothing here is medical advice.
To move the number down rather than explain it, see how to lower your resting heart rate.