Night sweats leave a trace: reading your wrist temperature data

If you have woken up in a tangle of damp sheets, kicked the duvet off, then lain there wide awake and a little bewildered — you are not imagining it, and you are not unwell for it. Night sweats are one of the most common experiences of the menopause transition; in large studies, most women go through hot flashes or night sweats at some point, often for years. For a long time they were something you just endured in the dark, with no proof they even happened. Now, on a newer Apple Watch, they leave a faint trace.

What a night sweat actually is

A night sweat is a hot flash that arrives while you sleep. It is a vasomotor symptom — a sudden, involuntary decision by your body to shed heat. Blood vessels near the skin open, you flush, your heart may pick up, and you sweat to cool down. It can pass in a couple of minutes, but it is often enough to surface you out of sleep and leave you cooling and clammy. None of this is a failure of willpower or a sign of weakness. It is thermoregulation, misfiring.

How your wrist quietly records it

On Apple Watch Series 8 and later, worn overnight in Sleep Focus, the watch samples your wrist temperature through the night. It does not read a night sweat and label it — nothing on your wrist knows what a hot flash is. What it captures is a relative trend: how warm this night ran compared with your recent nights. A night broken by a flush and a sweat can show up as a warmer-than-usual reading, sometimes with a more restless shape around it.

This matters because a single number is noisy, but a trend is not. Your overnight temperature genuinely moves with your hormones — in wearable research, nocturnal skin temperature ran about 0.30 °C higher in the luteal phase than the follicular phase, a shift far too small to feel but clear enough to measure across nights. You can read exactly what the sensor can and can’t do on the wrist temperature signal page.

The mechanism underneath the trace

There is real physiology here, not a hunch. Hot flashes are a thermoregulatory event. Your body keeps a comfortable “do-nothing” band of temperature — the thermoneutral zone — within which it neither sweats nor shivers. As estrogen falls, that band narrows dramatically. In a landmark study, the zone measured about 0.4 °C in women without hot flashes but collapsed to essentially zero in women who had them. When the tolerable band is that thin, a tiny rise in core temperature is enough to trip a flush and a sweat — which your wrist then records as a warmer night. The full mechanism is laid out in the research on the science page.

What the number can’t tell you

Your watch can show you that a night ran warm. It cannot tell you why, and it cannot tell you what it means for your health. Sheets, a hot room, a late glass of wine, illness, and plain ordinary variation all move the reading too. Wrist temperature is also relative, not absolute — it is a trend against your own nights, not a clinical thermometer, and it needs Series 8 or later plus a baseline of recent nights before it means anything at all. A run of warm, broken nights that leaves you drained is worth raising with your clinician; a single warm night usually is not. If the racing heart is part of your nights, that is its own thread worth reading.

How Perigee reads it

Here is the part that matters. Perigee never compares your temperature to other women or to a “good” number. It reads tonight against your own recent baseline, and it tells you how confident it is. So a warmer night shows up as exactly what it is — this night, a little hotter than your usual — with a plain-language note and an honest confidence level, not a score or a grade. When the data is thin, when you have only had the watch a few nights, or when the reading is noisy, it says so plainly rather than pretending to certainty.

One small thing

You do not need to fix anything tonight. But if the sweats are frequent, try keeping the bedroom genuinely cool and noticing whether the hot nights and the broken nights tend to land together — because when a symptom and a signal move in step, that is the pattern worth showing your clinician. Your watch has already been keeping the record. You just get to read it now.