Your wearable shows an HRV of 42 this morning. Yesterday it was 58. Are you broken? Probably not. Heart rate variability is one of the most misread metrics in consumer health tech, and the number on the screen tells you almost nothing in isolation. Trend, time-of-day, and context are the read.

What HRV actually measures

HRV is the variation in time between consecutive heartbeats, measured in milliseconds. A healthy heart does not beat like a metronome. It speeds up slightly on the inhale and slows on the exhale, responds to stress, digestion, sleep depth, and dozens of other inputs. That variability reflects the balance between your sympathetic nervous system (fight or flight) and parasympathetic nervous system (rest and digest).

Higher HRV generally indicates a nervous system that can flex between states. Lower HRV indicates a system stuck in sympathetic drive, which happens with acute stress, illness, poor sleep, alcohol, overtraining, or chronic load.

The number itself is not good or bad. A 35ms HRV in one person can reflect strong recovery. A 90ms HRV in another can reflect a bad night. Genetics, age, fitness, and sex all shift the baseline. Comparing your number to a friend's is a waste of time.

Why the absolute number lies

Whoop, Oura, and Garmin each calculate HRV differently. Whoop uses RMSSD during slow-wave sleep. Oura averages overnight readings. Garmin samples at varied points. The same heart will produce different numbers on different devices.

Even within one device, daily fluctuation of 20 to 30 percent is normal. A single low reading does not mean you are overtrained. A single high reading does not mean you are recovered. The signal lives in the rolling average, usually 7 to 30 days, and in how today compares to your own baseline.

If you only ever check today's HRV number, you are reading the weather. If you watch the trend across weeks, you start reading the climate.

What moves HRV up and down

The inputs are unglamorous and well established. Across our 1,380+ clients with BIA scans and tracked outcomes, the patterns are consistent.

HRV drops with:

  • Alcohol, even one or two standard drinks within four hours of sleep
  • Late meals, particularly large protein or fat loads close to bed
  • Poor or short sleep, especially fragmented sleep
  • Acute illness, often 24 to 48 hours before you feel symptoms
  • Heavy training without recovery
  • Psychological stress, deadlines, conflict, financial pressure
  • Dehydration and under-fuelling

HRV rises with:

  • Consistent sleep and wake times
  • Aerobic base training, Zone 2 in particular
  • Adequate protein and total calories
  • Nasal breathing and slow exhales
  • Time outdoors, sunlight in the first hour after waking
  • Resolved stressors

The unglamorous truth: HRV responds to the same things that drive almost every other longevity marker. There is no HRV hack separate from the basics.

How to read your trend

Start with a 30-day window. Most apps show this. Look for three things.

First, the baseline. Where does your HRV sit on a typical day with normal sleep and no alcohol? That is your number. Write it down. It will shift slowly as fitness and stress load change.

Second, the deviations. A drop of 15 to 20 percent below your baseline for one day is noise. A drop sustained across three to five days is signal. Something is loading you: illness, training, stress, alcohol pattern, sleep debt.

Third, the response. After a known stressor, a hard session, a late night, a stressful week, how fast does your HRV return to baseline? Faster recovery is the goal. A nervous system that bounces back in a day is more resilient than one that takes a week.

Time of day matters. HRV measured during deep sleep is the most reliable signal. Spot readings during the day are noisier and reflect whatever you were doing 10 minutes prior.

Where HRV is useful, and where it is not

HRV is useful for:

  • Catching illness early, often 24 to 48 hours before symptoms
  • Quantifying alcohol cost, the readings do not lie
  • Tracking the effect of a training block or stress period
  • Reinforcing sleep and circadian habits with objective data

HRV is not useful for:

  • Diagnosing anything medical, see your GP for that
  • Comparing yourself to anyone else
  • Single-day decisions, "should I train today?" is better answered by how you feel, your sleep, and your training plan
  • Replacing blood markers, body composition, or VO2 max as longevity inputs

For clients working through performance or longevity goals, we treat HRV as one input alongside BIA scans, food data from our 2,846 food dataset, training load, and labs run with their GP. It is a useful corroborator. It is not the primary driver of decisions.

The honest limitation

Wearables make money from engagement. The daily score, the recovery percentage, the colour-coded ring, all designed to keep you checking. Some of this is helpful. Some of it pushes anxious people into more anxiety about a metric that fluctuates normally.

If checking your HRV makes you sleep worse, stop checking it daily. Look at the weekly average on Sundays. The data is still there. Your nervous system does not need another reason to fire sympathetic at 6am.

What to do this week

  • Find your 30-day HRV average on your device and write it down. That is your baseline, not the population average.
  • Track one variable for seven days: alcohol, late meals, or sleep timing. Watch what your HRV does.
  • Stop comparing your number to anyone else's. Different bodies, different devices, different baselines.
  • Add one Zone 2 session of 30 to 45 minutes. Aerobic base lifts HRV over weeks.
  • If your HRV has been trending down for two weeks with no obvious cause, book a GP visit and rule out illness, thyroid, or iron issues before assuming it is training load.