Why do Oura and WHOOP give different HRV numbers?

Because they measure different parts of the night. Both report RMSSD. Oura averages five-minute samples across your whole night’s sleep. WHOOP uses an average weighted toward your last stretch of deep sleep. HRV changes with sleep stage, so the same night gives two numbers.

People wearing both often assume one is wrong. Both can be working fine. Think of two people timing the same marathon. One reports your average pace for the whole race, the other your pace over the last few miles. Same runner, same day, two honest answers.

What we’d do: pick one as your main history and judge it against itself. If you wear both, check that they move in the same direction after the same nights, such as a drinks night or a rest week. Never average the two.

How each one works

Both descriptions come from a 2025 study that put both devices on the same people, each night, against a chest strap.

Which is closer to a chest strap?

In that study, 13 adults slept with several devices and a chest strap for 536 nights. Compared with the strap’s whole-night average, Oura Gen 4 was off by about 6 percent on average and WHOOP 4.0 by about 8 percent.

Two scientists from WHOOP replied that the test wasn’t a fair match. WHOOP’s number leans on late deep sleep, and the strap’s number covered the whole night, so some of the gap is the window and not the sensor. Both points hold: WHOOP agrees less with a whole-night average because it isn’t trying to be one.

For you, the test that counts is whether one device, worn every night, shows the dips and recoveries your nights explain. Both can.