SlowpulseA low reading is unsettling mostly because the number arrives without context. It helps to know that the list of things which reliably push HRV down is long, well documented, and mostly mundane.
What the evidence identifies
A 2024 narrative review of the factors influencing HRV groups them roughly like this:
- Time of day. HRV rises through the night and falls considerably through the morning hours. A morning reading and a 3am reading are not the same measurement.
- Age. Highest in young adulthood, decreasing nonlinearly from there.
- Alcohol. Acute intake lowers HRV; sustained heavy intake is associated with reductions too.
- Hard training and competition. High-intensity efforts lower it, at least in the short term.
- Stress. Mental and work-related stress are both associated with lower HRV.
- Smoking, including passive exposure, in a dose-dependent way.
- Body composition. Higher BMI is associated with lower HRV.
- Heat and noise. Both push it down.
- Shift work. Night shifts lower it, and the review notes a relationship with how many years of shift work someone has done.
Read that list and the striking thing is how much of it is simply life. A late glass of wine, a hot room, a hard session, a bad week at work, a night shift — these are not anomalies to be alarmed by, they are the normal inputs to a signal that is doing its job by responding to them.
Before you conclude anything, rule out the measurement
A great many apparent drops are artefacts. Before taking a reading at face value, check that it is comparable to the ones you are measuring it against:
- Was it taken at a similar time of day? Given the overnight pattern above, this one matters more than any other.
- Was the watch worn the same way, and worn at all for a comparable stretch?
- Is it the same metric from the same device? SDNN and RMSSD are not interchangeable.
- Is it a single reading or a trend? One point is not a direction.
What a drop is, and is not
A fall in HRV is information about the last day or two of your life. It is not a verdict on you.
The useful question is never “is this number low”, because low against what? It is “is this low for me, right now, compared with the last couple of weeks of my own readings” — and that question needs a baseline, which needs consistent wear, which is the least glamorous and most important part of the whole exercise.
Which is what Slowpulse is doing
Slowpulse keeps a rolling baseline of your own resting heart rate and HRV from Apple Health, prefers overnight HRV readings because they are quieter and comparable night to night, and only reacts when today's readings pull away from your own normal — with the result discounted for how much you have exerted yourself.
Sources
Related
Not a medical device. Does not diagnose, treat or monitor any condition.