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Can Apple Watch measure cortisol?

Updated November 5, 2026 · Reviewed against the sources listed below

What cortisol is, and how it's tested

Cortisol is a hormone made by your adrenal glands. It helps your body respond to stress, and its level rises and falls through the day, usually highest in the morning.

Because it's a hormone in your blood, measuring it needs a sample. According to MedlinePlus (US National Library of Medicine), a cortisol test uses one of three:

• Blood, drawn at a lab, often twice on the same day because levels change.
• Urine, usually all urine collected over 24 hours.
• Saliva, sometimes collected at home at set times, such as late at night.

No optical sensor on a wrist can do this.

What Apple Watch does measure

Apple Watch records several signals that your stress response changes:

Signal	What it is	How stress tends to move it
Heart rate variability (HRV)	The small variation in time between heartbeats. Apple reports it as SDNN, in milliseconds.	Tends to drop
Resting heart rate	Your heart rate when you're still	Tends to rise
Sleep	Duration and stages	Short or broken sleep often goes with lower HRV the next day
Respiratory rate	Breaths per minute, during sleep	Can rise

A 2018 review of 37 studies in Psychiatry Investigation found that HRV changed under stress in most of them, most often in the direction of lower parasympathetic ("rest and digest") activity. The authors concluded HRV can be used to assess stress objectively, while noting there is still no single accepted standard for measuring stress.

So what is a "cortisol score"?

Apps that show a cortisol or stress score on Apple Watch are estimating your stress response from the signals above. None of them read your cortisol. A good score should:

1. Compare you with your own normal. HRV varies a lot between people. A value that's low for one person is ordinary for another.
2. Admit when data is missing, instead of guessing.
3. Say clearly that it is an estimate.

If you think your cortisol is actually too high or too low, for example with symptoms of Cushing's syndrome or Addison's disease, a watch can't tell you. Ask a doctor about a cortisol test.