It's always so hard to answer questions about temperatures, because we all run at such different levels. That said - several gals in the office wear them to sleep in. And I know many of our customers do the same. In truth, I've napped in mine (I do love me a mid-afternoon nap after a hard a.m. workout ), have slept in them most of a night (as for the time trials) but have never slept in them overnight entirely.