Why is it so difficult to sleep when it’s hot?

Philip Gehrman of the Penn Sleep Center offers an explanation—and explores some other recent questions posed by sleep scholars.

Woman sleeping next to her fridge

As stimulating as the warmth of summer air can be, it can also throw a wrench in a sleep schedule. Cue the stirring in bed while hot and sweaty, focused on getting cooler any way you can.

But why is temperature so intertwined with our ability to sleep, anyway? 

Philip Gehrman, an associate professor of psychology in the Department of Psychiatry at the Perelman School of Medicine and member of the Penn Sleep Center, explains the link, plus, offers a few tips for easing into a deeper sleep during the scorching summer months.

Why does temperature matter so much to our bodies when we’re trying to sleep?

There are two levels of it. At a surface level, it’s a comfort issue. The more of a discrepancy there is between our body temperature and room temperature, whether it’s too cold or too hot, it’s just harder to get comfortable and fall asleep. 

At a deeper level, there’s actually a pretty complex relationship between temperature regulation and sleep regulation. And so, our body temperature, in particular our core body temperature, changes over the course of the day in predictable ways, and there are relationships between our core body temperature and how sleepy we feel at any point.

And what is that relationship?

What happens is there’s a regular daily rhythm, called the circadian rhythm, our core body temperature. Our core body temperature is lowest roughly two to three hours before our natural wake-up time in the morning, and then increases over the course of the day and peaks about two hours or so before we start to feel sleepy at night. What happens is that after our core body temperature peaks, and then starts to drop, that cooling of the core tends to make us feel sleepy. This is why if you’ve ever sat in a hot tub or taken a hot bath, afterwards, people often feel sleepy when they get out. And people assume it’s because they’re very relaxed. That’s part of it, but a bigger factor seems to be, you sit in that hot water, it raises your core temperature, then you get out and your core temperature rapidly drops, and it’s the cooling of the core that tends to make us feel sleepy.

Is this why some people recommend a hot shower or bath before bed?

A bath, yes. Showers, not really, because if your body is not immersed in the water it doesn’t have nearly the same impact on the core. It can still be very relaxing. But it doesn’t have the same effect.

Have you heard that feet have a role in regulating body temperature? Anything to that? 

When I tell people that the body cools down at night, they say, ‘But I feel warm, hotter at night, not cooler.’ What happens is the heat produced in our core by our organs gets distributed to the surface of our skin, to then dissipate into the air, that’s how our core cools. But it’s the temperature at the surface of the skin we feel. We may feel hotter at night, but it’s because our core is redistributing heat from the core to the surface, to the periphery. And one of the places of the body where we’re best able to dissipate that heat may be the feet. It may be that if people wear socks at night, for example, it’s harder to release that body temperature. That’s speculative but could be the case.

If it is a hot summer day and you’re asleep, does that affect REM, dreaming, the quality of your sleep?

I don’t think we know. Certainly, I would expect that if our body temperature is not able to sufficiently cool at night, it could keep us in lighter stages of sleep. Again, that’s extrapolating a bit, but I could imagine that being the case.

What is an ideal sleep environment when it’s hot? 

Certainly, we know you get the most deep sleep in a cool bedroom—not cold, but cool. The problem is what’s cool to one person is not cool to another person. There are a lot of individual differences in this. And so, it’s hard to give a particular temperature, but for each individual the optimal temperature would be a cool bedroom.

What about bedding material?

Despite the claims from companies, there’s no evidence certain materials are better than others. It’s a lack of research … but it doesn’t [seem to] make a difference. I can’t give specific guidelines because we don’t have the evidence.

Does drinking water cool you down?

Not that I’m aware of. And, of course, you wouldn’t want to drink too much liquid before bed because you’ll wake up more often.

What about fans and misters?

They can be very helpful. A lot of people in the summer like to have a fan running for the temperature, but also that background noise can be soothing as well.

Where do you think we are—it’s fascinating, as much as we know about sleep there’s so much we seem to not know. What is the state of sleep study right now? What is the progress?

There’s been tremendous progress since I first got in the field in the mid-‘90s; we knew almost nothing then. We know a lot more but there’s still a huge amount we don’t know. 

For example, we don’t have a good sense of what good quality sleep is. You can ask people, ‘Do you feel you slept deeply last night?’ But how do you quantify that? What’s the biology of a good night of sleep versus a bad night of sleep? It’s a basic question, but we really don’t have a good sense of that. 

A hot area of research, in the last few years, is there’s growing evidence of a link between inadequate sleep and a risk for dementia that people are trying to understand. For a while, a big area of research has been looking at the relationship between sleep and learning and memory. We’ve learned that when you learn information during the day, it’s during sleep that new synaptic connections are solidified. Lack of sleep can interrupt that process. I always tell my students one of the worst things you could do before an exam is pull an all-nighter. 

There’s lots of evidence now that sleep is really important for physical and mental health. I don’t think anyone will argue with that. But understanding the mechanisms of the connections, ‘What is it about sleep that’s important for health?’ That’s what a lot of people are trying to figure out right now.