Wrist Temperature: Your Body's Hidden Signal

Wrist temperature is a sleep metric available on Apple Watch Series 8 and later. It measures how your overnight skin surface temperature deviates from your personal baseline, revealing subtle changes happening inside your body.

Why Does Wrist Temperature Matter?

Your body temperature isn't a constant 37°C (98.6°F) — it follows a circadian rhythm. When you fall asleep, your core temperature drops while your extremities (including your wrist) warm up. This is your body's natural way of dissipating heat to initiate sleep.

Deviations in wrist temperature can reveal:

• Hormonal cycle changes: Body temperature naturally rises about 0.2–0.5°C after ovulation
• Immune system activity: Temperature may shift upward before a cold or infection sets in
• Lifestyle factors: Alcohol, intense exercise, and ambient temperature can all affect readings

How to Read the Deviation

Mellows shows the deviation from your personal baseline rather than your absolute body temperature:

• Near 0°C: Normal range — your thermoregulation is stable
• Positive deviation (+0.2°C or more): Above baseline — may relate to hormonal changes, immune response, or a warm sleep environment
• Negative deviation (-0.2°C or more): Below baseline — may relate to a cold environment or a loosely worn watch

Fluctuations within ±0.5°C are typically normal and not cause for concern.

What Affects Wrist Temperature?

• Increases: Ovulation phase, infection or inflammation, alcohol, warm environment, heavy bedding
• Decreases: Cold environment, loosely worn watch, follicular phase of the menstrual cycle

Using This Data in Mellows

Mellows tracks your nightly wrist temperature deviation and compares it against your personal baseline. Combined with HRV, heart rate, and other metrics, it provides a more complete picture of your recovery status. When multiple indicators shift simultaneously, it often signals that your body needs extra attention.

Wrist temperature requires Apple Watch Series 8 or later and needs at least 5 nights of wear to establish a personal baseline.