What is actually happening
Your brain keeps core body temperature inside a narrow comfortable band. During perimenopause, changing oestrogen levels affect the part of the hypothalamus that sets that band, and the comfortable zone becomes narrower. A small rise in core temperature that you would normally not notice is now read as too hot, so the body reacts to shed heat: blood vessels near the skin widen, you flush, and you sweat. The sweat cools you, sometimes below comfortable, which is why a night sweat can end in a chill (Journal of Clinical Endocrinology & Metabolism, 2021).
Because the whole episode is a swing in body temperature, it can leave a mark in your skin temperature overnight, even on the nights you sleep through it.
How common, and how long
Vasomotor symptoms, the medical name for hot flushes and night sweats, are the most common symptom of the transition. In a European survey, between 31% and 52% of women reported them at moderate to severe intensity, depending on the country (Maturitas, 2023). They are not a brief phase for everyone either. For many women they last several years, and for some a decade or more, often continuing past the final period (Journal of Clinical Endocrinology & Metabolism, 2021).
Why they affect sleep so much
A night sweat does not simply make you hot. It tends to coincide with waking, so even when you do not fully remember the episode, your night has been broken into pieces (Journal of Clinical Endocrinology & Metabolism, 2021). That is why the tiredness from night sweats often feels out of proportion to how many you can actually recall, and why perimenopause and sleep are so tightly linked.
When to talk to a doctor
Night sweats are usually part of the transition, but they are worth raising with a doctor if they are affecting your sleep or daily life, because effective treatments exist. It is also worth a check if they are drenching, come with a fever, weight loss, or feeling unwell, or do not fit the usual picture, since sweating at night has other causes too. anna does not diagnose, and this article is not medical advice.