There are so few online reviews for Slumbercaps (
http://www.slumbercap.com/) that I have to add in my personal experience.
As a newly bald man, I was truly shocked at how cold my head would get while sleeping during the winter months. I didn't want to resort to a regular knit cap because I knew I'd end up sweating and I figured that the bulkiness wouldn't really be comfortable. I had to do something because my wife didn't really appreciate me continually stealing the sheets to tuck in over my head.
I took the advice of a January 2008 SBG blog entry (
http://www.slybaldguys.com/serendipity/archives/176-Slumbercap-how-to-keep-your-head-warm-while-sleeping.html) and requested one of the women's size for Christmas 2009. Well, I got two black caps and they're great! My concerns about sizing (I have a large head) proved to be unfounded and they fit perfectly - not too tight but not loose either. It's a soft material that's warm yet breathable. Haven't woke up with any head sweating yet. I do recommend the women's size unless you really want to go for the old-timey long stocking cap look ... apparently, the length of the cap is the only difference between the sizes.
In short, I'm extremely pleased with this product and I give Slumbercaps a emphatic two thumbs up!