CircadiaOSShort answer: the most common causes are a bedroom that is too warm for the first half of the night, bedding or a mattress that traps heat, a partner's body heat, and alcohol or a heavy meal late in the evening. Hormonal changes (perimenopause, the menstrual cycle, pregnancy), some medicines and some medical conditions can also cause it. If you are soaking the sheets, or it comes with fever or weight loss, see a doctor.
What your body is doing at night
To fall asleep and reach deep sleep, your core temperature has to drop about 1–2°F. It does that by pushing heat out through your skin, especially your hands and feet (Kräuchi, 2007). If the room, the bedding or the mattress stops that heat from leaving, your skin warms up, you get restless and you wake. Deep sleep is concentrated in the first half of the night, which is why many people wake hot a couple of hours after falling asleep rather than at dawn.
Nine common causes
The room and the bed
- The thermostat is set for the evening, not the night. A room that feels fine at 10 p.m. with the lights on can be too warm once you are under the covers. Most sleep guidance points to about 60–67°F.
- Bedding that holds heat. Polyester sheets, a heavy comforter, or a memory foam mattress without a breathable cover all keep warmth against your body.
- Sharing a bed. A sleeping adult gives off roughly as much heat as an old light bulb. Two people under one duvet adds up. More on sleeping hot next to a partner.
- Humidity. Humid heat disrupts sleep more than dry heat, because sweat cannot evaporate (Okamoto-Mizuno and Mizuno, 2012).
The evening
- Alcohol. It widens blood vessels in the skin and fragments sleep in the second half of the night.
- A late heavy meal, spicy food or late workout. All raise body heat for a few hours.
Your body
- Hormones. Perimenopause and menopause are the best-known cause of night sweats. Temperature also runs higher in the second half of the menstrual cycle and during pregnancy. Menopause and the thermostat.
- Medicines. Some antidepressants, steroids, diabetes medicines and hormone treatments list sweating as a side effect (NHS; Mayo Clinic).
- Medical conditions. Anxiety, an overactive thyroid, infections, low blood sugar and, rarely, some cancers can cause night sweats. Night sweats when it isn't menopause.
How to narrow it down
- Check the actual bedroom temperature with a thermometer by the bed, not the hallway thermostat.
- Note when you wake. Waking hot one to three hours after falling asleep points to the room or bedding during the deep-sleep window. Drenching sweats at any hour point toward something else.
- Change one thing for a week. Cooler first half of the night, or lighter bedding, or no alcohol. Not all three at once.
- Try a curve, not a lower number. Turning the thermostat way down often swaps waking hot at 1 a.m. for waking cold at 5 a.m. Cool early, warmer before you wake. How to set that up.
When to see a doctor
Talk to a doctor if you regularly wake with sheets or clothes soaked through, if the sweating is new and you cannot explain it, or if it comes with a fever, unexplained weight loss, a cough that will not go away, or swollen glands. A cooler bedroom will not fix those, and they are worth checking.
Where CircadiaOS fits
If the cause is the room, CircadiaOS can take over the curve. It is an iPhone app that sends a four-phase overnight schedule to the smart thermostat you already own (Nest, ecobee, Honeywell Home and 20+ more brands) every evening: cooler as you fall asleep, coolest in early night, a little warmer later, warmest before you wake. You can move the whole curve cooler, down to 63°F. On Premium it reads your wearable's sleep data and adjusts one phase at a time, keeping only changes your data supports. It is a wellness app, not a medical device, and it does not diagnose or treat night sweats or anything else.
Questions
Why do I wake up hot at night but I'm cold when I go to bed?
Your body warms up under the covers and releases heat through your skin as you fall asleep. If the bedding holds that heat, or the room stays at an evening temperature, you overheat a couple of hours in. Lighter layers and a cooler first half of the night usually help.
What temperature should my bedroom be if I wake up hot?
Try the lower end of the usual 60–67°F range for the first half of the night, then let the room warm 2–3°F before you wake so you do not swap waking hot for waking cold.
Is waking up hot a sign of something serious?
Usually not. Most cases come down to the room, bedding, alcohol or hormones. See a doctor if you are drenching your sheets, if it is new and unexplained, or if it comes with fever, weight loss or other symptoms.
Does alcohol make you wake up hot?
It can. Alcohol widens blood vessels near the skin and disrupts sleep in the second half of the night, so many people wake warm and restless after drinking in the evening.
Can a smart thermostat stop me waking up hot?
It cannot promise that, but it can remove one common cause: a room held at one temperature all night. A schedule that is coolest in the first half of the night and warmer toward morning is easy to set on most smart thermostats, or CircadiaOS can send one each evening.
Sources
- NHS, "Night sweats" (nhs.uk/symptoms/night-sweats).
- Mayo Clinic, "Night sweats: causes" (mayoclinic.org).
- Kräuchi K. The thermophysiological cascade leading to sleep initiation in relation to phase of entrainment. Sleep Medicine Reviews, 2007.
- Okamoto-Mizuno K, Mizuno K. Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology, 2012.
- National Sleep Foundation, "Sleep tips" (thensf.org/sleep-tips).
- Cleveland Clinic, "What's the best temperature for sleep?" (health.clevelandclinic.org).
Related guides
- How to sleep cooler — nine practical fixes.
- For hot sleepers — settings when you run hot.
- Night sweats when it isn't menopause — other causes and when to get checked.
- Why do I wake up at 3am? — the other end of the night.
- Best temperature for sleep — the research.