Searching for the Perfect Thread Count

After spending 18 months designing a bed sheet, I discovered a few things about the world’s greatest textile.

I led the development of the Casper bed sheets, and during the research phase, what we often heard from others in the industry was that a high thread count made sheets softer and better quality.

It’s the kind of story that kids interrupt because it doesn’t sound right. The idea is that a really high number of threads in every square inch — which is what thread count refers to — means there are fewer gaps between threads, and the softer and longer lasting the bed sheets will be. It’s unclear where this idea comes from. It’s mostly just a tale. The real impact of thread count is on temperature and relative humidity as we sleep.

Thread count doesn’t really speak to softness. During our R&D, we had research participants test high, medium, and low thread count sheets and rate them based on subjective softness. There was no pattern between the thread count of the sheets and their softness ratings. If anything, the opposite was actually true, where thread count had an inverse relationship with people’s subjective measures. (This was one of the more fun tests to run.)

What thread count did impact was airflow. A sheet with a weave of 100 x 100 has 100 vertical threads and 100 horizontal threads for a total count of 200 threads per square inch. That is already almost four threads every millimeter. If you held a sheet like this up and did your best genie impression and blew colored smoke through it, some would slowly trickle out the other side. At 1,000 threads per square inch, which a lot of sheets are marketed as, air barely can pass through the weave.

It can be especially important for people who tend to sleep hot. A set of sheets with a high thread count will excessively trap the heat and moisture your body gives off as you sleep. Using sensors and some specially-designed experiments, we found that the relative humidity under the covers can reach 80% during the night, depending on the person, and the thread count (density), of the sheets they’re sleeping under. A high thread count sheet is like oversleeping in a tent, long after the sun has come up.

The right thread count helps your body to do what it already does well: regulate its own temperature. After many rounds of feedback, we zeroed in on an optimal number of threads per square inch (400) that helps the body to maintain a comfortable temperature during the night. The resulting set is designed to be crisp, natural, and support better rest. Unlike a lot of sheets, too, they improve over time. You can see how they came out here.