Do hot flashes wake you, or does waking bring them?

If a hot flash wakes you up most nights, it is natural to blame the heat for the broken sleep. The National Institute on Aging adds a twist: it says research now suggests that waking from sleep may itself trigger hot flashes, "rather than the other way around". This page goes through the studies behind that sentence, what they found about the order of events, and what it changes about how you handle the night.

How researchers time a flash against a waking

You cannot answer this question from memory. By the time you notice the heat you are already awake, so you cannot tell whether the flash came a minute before or a minute after. Sleep labs get round it with two recordings running side by side. Brain-wave sensors (polysomnography) mark the exact 30-second stretch in which a woman goes from sleep to waking. A pair of small electrodes on the breastbone measures skin conductance, which jumps when sweating starts, and that jump marks the start of a flash. Line the two up and you can see which came first.

What the sleep-lab studies found

One of the first careful looks came from the sleep researchers Freedman and Roehrs. Among awakenings that fell within two minutes of a hot flash, 55% began before the flash. They concluded that hot flashes did not disturb sleep in the women they studied. In a follow-up they refined that view: flashes did seem to break sleep in the first half of the night, when awakenings were more likely to follow a flash, but in the second half that order no longer held. Both findings are summarised in the introduction of a 2014 paper in Fertility and Sterility, and they are the research NIA is pointing to.

That 2014 study, from SRI International in California, then measured the size of the problem in 34 perimenopausal women over 63 lab nights. The sensors picked up 3.5 flashes a night on average, and 69.4% of them came with an awakening that started within a minute before to two minutes after the flash. Waking linked to flashes added up to about 16.6 minutes a night, roughly a quarter of all the time the women spent awake after first falling asleep. The spread was wide, from none of a night's waking to 89% of it.

A 2016 study in the Journal of Clinical Sleep Medicine came at it differently. Researchers at Harvard gave 28 healthy younger women a drug that switched off their ovaries for a few weeks, so hot flashes appeared without the other sleep changes of midlife. Two thirds of the night flashes fell within five minutes of an awakening. Of those, 38% started before the waking, 42% at the same moment, and only 20% after it, with no difference between the two halves of the night.

Put together, the studies agree that flashes and wakings are tightly bound. They disagree on how often the waking comes first, and the difference probably reflects who was studied: women in midlife have more early-morning waking for other reasons, so more of their flashes may be set off by a waking that was coming anyway.

Why a hot flash wakes you up

A hot flash is the body getting rid of heat. The 2014 paper describes it as the blood vessels near the skin widening while sweat breaks out on the upper body. The Harvard authors list two ways that could break sleep: the discomfort of sudden sweating, and a small rise in core temperature, up to about 0.1°C, that usually comes just before a flash. Either one can pull you up out of light sleep. Most flashes in the SRI study began in stage 2 or deep sleep, so many of them did more than nudge a sleeper who was half awake already.

About one in five recorded flashes passed without waking anyone. That fits with what the SRI women reported in the morning: about 1.5 flashes a night against the 3.5 the sensors recorded.

Waking up with hot flashes later in the night

In the second half of the night the picture changes. Deep sleep is mostly used up, sleep is lighter, and you surface more easily for any reason: a full bladder, a partner turning over, the habit of waking at the same hour. If the flash then follows the waking, the heat is something that happened to you once you were up, rather than the thing that woke you. The SRI study found flashes peaked in the fifth hour after lights out, which for many women is the stretch between two and four. Our article on waking at 3am in menopause covers that hour in detail.

The Harvard team noticed something else worth knowing. The flashes women remembered in the morning tracked how broken their sleep had been more closely than the flashes the sensors recorded. Their reading is that a broken night makes you more aware of a flash and more likely to remember it. So a night that felt full of flashes may have been a night full of wakings, with the flashes the part you recall.

What this means for how you handle the night

If the flash wakes you, cooling the bed is the obvious lever, and it is most likely to pay off in the first few hours of sleep. NIA's list for night flashes includes a cooler room, a fan, light layered bedding you can push off and a sip of cold water before bed. The bedroom checklist for hot nights puts those steps on one page.

If you wake first and the heat follows, a cooler room still makes the flash easier to sit through, but the waking itself needs other work. NIA's sleep habits apply here: the same bedtime and getting-up time every day, no naps late in the day, caffeine early, alcohol not used as a sleep aid. NIA also says cognitive behavioural therapy for insomnia (CBT-I) has been shown to help women with menopausal symptoms sleep better, and menopause insomnia and how long it lasts explains what that therapy involves. A racing heart or a rush of worry can be part of the same moment, which anxiety at night in perimenopause takes up.

Note which came first

You cannot wire yourself up like a lab, but you can notice roughly what happened. When you wake, ask yourself two things before you check the clock: was I already hot and damp when I came to, or did the heat build after I was awake? And what else was going on: a full bladder, a noise, a dream, nothing? Write the time and the answer down in the morning. The printable night diary has a row for each waking and a column for whether you were hot before or after.

Two weeks of notes usually show a pattern. Heat first, early in the night, points to the bedroom. Waking first, at the same hour whether or not you were hot, points to sleep habits, the bladder or worry.

When to talk to a doctor

The NHS says to see a GP if night sweats are regular and wake you up, or if they come with a high temperature, a cough, diarrhoea or weight loss you cannot explain. The same goes for wakings most nights that leave you worn out by day. If you are unsure whether yours need a visit, our night sweats warning-signs checker goes through the signs one by one. Ask about treatment for the flashes themselves at that appointment. This site gives no advice on HRT, medicines or supplements.

My take

I'd start with the timing of your own nights rather than the question in the abstract. The lab evidence says early-night wakings are more often the flash's doing, while a 3am waking may have come first and brought the heat with it. My pick for week one would be a cool, layered bed for the early hours, and for week two a fixed wake time every morning. I'd judge each by the diary: whichever week cut the wakings is the one to keep. Everything else on night heat is collected in the hot flashes at night section, and soaking sweats in particular in night sweats.