Short version: the stress monitor reads your heart, continuously, and scores arousal — heart rate up while heart-rate variability drops. That combination is the cardiac signature of your autonomic nervous system allocating resources.
It is a genuinely useful thing to measure. It is also widely misread, so it is worth being precise about what the number contains and what it cannot.
What’s in the number
Two inputs, both from the optical heart sensor: heart rate, and the beat-to-beat variation between heartbeats (HRV). Arousal pushes them in opposite directions at once — rate up, variability down — and the score tracks that divergence through the day. Because a strap is worn continuously, it catches this at all hours, not just during workouts. That continuous coverage is the real strength of the form factor.
What’s deliberately not in the number
Content. The signal carries no information about what you were doing. This is not a Whoop limitation — every wrist wearable’s stress feature works from the same cardiac ingredients, and none of them can see your day. A nervous first date and a difficult meeting are indistinguishable at the wrist. So are a brutal Monday and a brilliant one.
Whoop is straightforward about this. Their team’s public framing: stress is neither good nor bad — it is your body’s level of activation, which can be appropriate or not depending on the situation. Only you know the situation.
The two-sentence version worth remembering
Activation means something happened. It cannot tell you what mattered.
The score is honest about the first half. Every misreading of it — panicking over a high afternoon, celebrating a “calm” sick day on the couch — comes from asking it for the second half, which was never in the signal.
Where the context comes from
Whoop’s own recommended workflow says the quiet part plainly: look at the day’s stress graph, find the windows that ran high, then go check your calendar and think back to what you were doing in them. The calendar is the missing column — it is what turns “elevated, 2 to 4 pm” into “the quarterly review ran long.”
You can run that loop by hand, and it works. brainful exists to run it automatically: it reads the same live broadcast the strap already sends and scores each calendar event 0–100 — the b-score — so the number arrives already attached to the hour it describes. Either way, the principle is the same: the signal tells you that; your schedule tells you what; and only the two together tell you whether it was worth it.
Source: WHOOP’s public documentation and their team’s public Q&A on the stress monitor. The characterization of stress as a “level of activation” and the check-your-calendar workflow are WHOOP’s own.