Exploring the Science of Sleeping Hot

Why do we go to sleep—feeling perfectly comfortable—but sometimes wake up hot and sweaty hours later?

There’s nothing like slipping under the covers after a marathon day. We pull the sheets up around our shoulders and our thoughts recede. Sometimes we wake up a few hours later, hot and sweaty, and have to flush the covers to cool down. It’s something we’ve come to accept as normal.

Why does waking up sweaty ever have to be normal? There’s some biology behind the idea of being a “hot sleeper,” but there’s another big reason. Our body is pretty good at regulating its own temperature but certain sheets and mattresses, because of how they’re made, challenge the body’s natural efforts and cause us to sweat. It’s a common reason we sometimes wake up feeling like we’re in Miami. As a mechanical engineer at Casper, and someone who naturally sleeps hot, I set up a small experiment to better understand why.

Typical bedding products can cause wild swings in the environment around us. Our body temperature changes during the night, shedding heat as our core temperature drops, and a lot of products we sleep with lock in this heat (and moisture) rather than help regulate it. The top layer of many mattresses, for example, blocks air from moving away from the body, depending on the design. A lot of bed sheets have a thread count that is much higher than it needs to be, meaning they’re incredibly dense. The effect on how we sleep can be dramatic.

I was curious to see this first hand, so I set up a small study. I designed lightweight sensors and wore them on my back and each foot. I also asked some colleagues and friends to do the same. We pulled some generic bedding up around our shoulders and closed our eyes, and what emerged through the data was a picture of how the environment we fall asleep in can quickly change.

These are common graphs from a night of mine. After getting into bed, the temperature under the covers climbed about 25 degrees and hovered around 95 degrees throughout the night, while the relative humidity fluctuated to uncomfortable extremes. At certain points, the environment under the sheets was similar to a muggy summer day in Houston.

The wild swings were interruptive. If you look at the relative humidity graph, you can see that the moisture in the air climbed aggressively and then plunged, around 4:30 am. In my notes the next morning, I wrote that I woke up sweaty and had to flush the covers with fresh air, because the high humidity, paired with the elevated temperature, felt awful. Throughout my research, I observed events like these to be a regular occurrence.

These trends were fairly typical of what we saw with others’ data. Even if the room was at the ‘perfect’ sleep temperature of 65–68 degrees and didn’t change, there were significant fluctuations in temperature and relative humidity under the covers. As an engineer at Casper, I’ve been thinking a lot about how we design for this.

Insights like these have been driving a lot of mine and my teammate’s work. We recognize that the body is good at keeping itself at the right temperature during sleep, so we’ve been creating products that encourage it. For example, we chose a particular fiber, thread count, and weave for the Casper sheets that help the body stay at a nice California warm during the night. The Casper mattress utilizes a specially-designed upper layer that helps to balance heat and move it away from the body. We’ve been developing an entire system that supports the body in sleeping naturally cool, and we’re seeing encouraging data on how it impacts our rest.

Because even if there aren’t any gold medals for sleeping, I want to wake up feeling like I just won one, and shouldn’t we all.

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