What is the difference between HRV measured at night versus during the day?

Overnight resting HRV (measured during sleep) is the clinically meaningful value, daytime HRV is contaminated by movement, respiration rate changes, posture, stress reactivity, and digestion, making it unstable and difficult to interpret (Shaffer & Ginsberg, Front Public Health, 2017).

All major consumer wearables (Whoop, Oura, Garmin, Apple Watch) default to overnight measurement for this reason. The overnight value, particularly during deep sleep when the parasympathetic system is most active, gives you the cleanest signal of baseline autonomic tone. Daytime HRV measurements can be useful for specific purposes, measuring acute recovery from exercise, or doing biofeedback sessions with a Polar H10 chest strap, but day-to-day comparison against your baseline should use the overnight figure. When your device shows you a "morning HRV," it is typically derived from the last 5-minute deep sleep window before waking.

Honesty Scale: Solid (1). Sleep-based HRV measurement is the clinical standard for wearable autonomic assessment.

What to do: Stop comparing your app's daytime spot-checks to your overnight trend, they are measuring different things. If you want a consistent morning reading, do it with a Polar H10 and a Kubios app during 5 minutes of silent supine breathing immediately after waking, which gives a value comparable to your overnight measurement.

For the full picture, read Your Whoop Is Worried. Here Is What It Actually Found.