Why do my feet get hot as I fall asleep, and why do I wake up hot later?

Host: Why do my feet get so hot right when I'm falling asleep?

Circadian: Because that is your body doing the most important job of the evening. Your core temperature falls at night, and the drop is part of the daily cycle your body clock runs. The Sleep Foundation puts the start of it at roughly two hours before bed. To lower your core, your body has to move heat out, and it does that mostly through your hands and feet. The blood vessels there open up, warm blood comes to the surface, and the heat leaves through the skin. Hot feet at lights out mean that system is running.

Host: So hot feet are a good sign?

Circadian: They are a normal sign. Sleep researchers track the gap between skin temperature far from the heart, at the hands and feet, and skin temperature closer in. A 2011 review in Pflugers Archiv reports that in controlled studies that gap is the best available marker of how fast sleep arrives. I should add the limit on that. Studies in ordinary bedrooms have not always matched the lab results, so it is a strong signal in controlled conditions and a softer one at home. Either way, the thing they measure is heat moving out of you.

Host: Then why do I wake up hot in the middle of the night, hours after my body supposedly cooled itself down?

Circadian: Because your body can only send heat somewhere that will take it. Think of the heat leaving your skin as needing a place to go. If the air and the bedding around you are cooler than your skin, the heat moves out easily. If they are close to your skin temperature, it barely moves, and it builds up against you. So the question to ask when you wake up hot is what stands between your skin and the air.

Host: And I'm guessing that's the bed, mostly.

Circadian: The bed first, in a particular order. The mattress is the biggest surface and the hardest to change, then the comforter, then the pillow. Memory foam is the clearest example of a material that holds heat. It is a solid block with small sealed pockets, so air does not travel through it, and it softens as it warms. Your heat sits in the dent your body made, and that dent gets warmer as the night goes on. Polyester does a similar thing inside a comforter in a different way. The fiber takes on almost no moisture vapor, so what your skin gives off stays right where it landed, and you feel a clammy warmth under the cover.

Host: I bought a cooling gel pillow for exactly this. It felt amazing for about ten minutes.

Circadian: That is how gel behaves. It starts colder than you, so the first contact feels great. Then it takes on your heat until it matches you, and from there it is a warm surface you cannot cool back down. A cold layer on top of a warm block does not change the block, and the block decides how the night goes. People make that mistake a lot: they buy a cooling product and leave the hot one underneath it.

Host: Okay, but I'm not replacing my mattress because my feet are warm. That's a lot of money for a theory.

Circadian: You should not start there, and I would not tell you to. Start with the room, because it costs nothing. The Sleep Foundation puts the comfortable band at 65 to 68 degrees Fahrenheit and calls roughly 65 the best of it. That sounds cold when you read it. It sleeps well, because your skin needs somewhere cooler than itself to send heat, and a warm room shrinks that gap. A 2012 review by Okamoto-Mizuno and Mizuno in the Journal of Physiological Anthropology reports that with ordinary covers, heat costs more sleep than cold does. More waking, and less deep sleep, in younger and older adults alike.

Host: What if the thermostat is already down and I still wake up hot?

Circadian: Then check the humidity. Wet air slows evaporation from your skin, so a humid room feels hotter than the thermostat reads, and a dehumidifier is often the cheaper next move. After that, look at what else adds heat. A second person under the same comforter is a second heat source under one lid, and the person on the inside of the bed gets the worst of it. Two lighter comforters on one bed tend to solve that better than one heavy one, because each person keeps their own covers and their own air. A drink before bed moves your heat forward: alcohol opens the vessels near the skin, heat leaves early, and later in the night that reverses, so the second half runs hot. And some common prescriptions raise body heat or bring on sweating as a side effect.

Host: And if none of that explains it, and I'm still waking up hot?

Circadian: Then it may not be a bedding question at all. Hormone changes in your forties and fifties are the cause people spend the longest trying to buy their way out of. Perimenopause and menopause bring hot flashes, and a hot flash during sleep is a night sweat. Pregnancy and thyroid conditions move temperature too. A clinician can tell those apart from an infection or a medication effect and treat the actual cause. MedlinePlus lists sweating often during sleep, or losing weight from sweating, as reasons to contact your provider. Sweat that soaks a sheet, comes with a fever, or comes with weight loss you did not intend belongs with a doctor before it belongs in a store.

Host: So where does bedding come in, if it does at all?

Circadian: Bedding decides one thing, which is whether the heat you produce can leave. No pillow or comforter changes your body clock, and I would be wary of anyone who says theirs does. What the material can do is let heat and moisture through or hold them against you. Cotton is a dense fiber in an open structure, so air keeps moving through it. Wool takes on moisture vapor, up to about 30 percent of its own weight before the surface reads wet, and your heat leaves with that vapor, so the surface you lie on does not turn into a hot spot. Kapok is a hollow fiber, 80 to 90 percent air by volume, so air travels through the whole pillow. Buckwheat hulls leave thousands of open channels that carry warm air up and out.

Host: If I could only change one thing on the bed, which would it be?

Circadian: The comforter, because it covers most of you. The pillow matters because your face sits right on it all night with no cover to kick off, but your head is a small share of your skin. And if you have a partner, the two-comforter idea often does more than any change of material.

Host: So the order is the room, then the extras like a partner or a drink, then the comforter, and the pillow last.

Circadian: That is the order I would use. Your body is already trying to cool you down every night. Most of the fix is getting out of its way, and the cheapest steps come first. If you want the details and the studies behind each step, the full transcript and the written guide are on circadianrest.com.