Exploring the Science of “Sleeping Hot”

By Jordan Lay  |  Jan 9, 2017
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Why does anyone wake up sweaty?

There’s nothing quite like slipping into bed. Cool at first, we begin to warm up under the sheets and arrive at the perfect temperature just in time to fall asleep. But the environment continues to heat up as the hours tick on. At some point many of us start to get sweaty. Why?

The reason people “sleep hot” has a lot to do with design. Our core temperature drops by a couple degrees during the night, shedding heat into the surrounding areas, and certain sheets and mattresses trap the heat and moisture in around us. This is especially true of memory foam mattresses and high-thread-count sheets (which are incredibly dense). We see the impact in tests we run in our California lab. Bedding design is a common reason people sometimes wake up feeling like they’re in Miami.

As someone who naturally sleeps hot, I set up an experiment to better understand what’s happening under the covers. I designed custom temperature and humidity sensors and wore them on my back and feet for several weeks. Then I pulled some generic bedding up around my shoulders and drifted off to sleep. What emerged through the data was a picture of a “hot sleeper.” Here’s a typical night of mine.

The temperature under the covers fluctuated significantly. What I found is that it skyrocketed soon after bedtime (climbing about 25 degrees) and then stayed within a certain band over the course of the night (hovering around 95 degrees). However, this wasn’t all that dramatic.

The relative humidity is where we see big swings. If you look at the relative humidity graph, from the same night, you can see that the moisture in the air jumped up and down. It climbed aggressively from midnight until about 3:00 a.m. and then plunged. What happened is 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. The environment forced me to wake up.

Throughout my research, I’ve seen this be a common problem for people. Even if the room is at the ‘perfect’ sleep temperature of 65–68 degrees and doesn’t change, there are significant fluctuations in relative humidity under the covers; it’s often caused by dense bedding products that restrict proper airflow. As a mechanical engineer at Casper — and a hot sleeper — we’ve incorporated a lot of these learnings into the products we design.

The body is good at keeping itself at the right temperature during sleep, so we create products that help support it in sleeping naturally. We designed the Casper sheets and pillow to promote balanced airflow by focusing on just the right thread count, weave, and fibers. We also engineered a custom layer for the Casper mattress that helps to balance heat and move it away from the body. We’re even developing an entire system that supports the body in sleeping naturally cool, and we’re seeing encouraging data on how it impacts our rest. It’s all part of our product philosophy.

Jordan Lay is a senior mechanical engineer at Casper. He specializes in concept development and consumer products.

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