Heavy and thick shirt, it's half wool meaning 50 acrylic 50 wool (that's what the label says) but it feels like full wool. It keeps the body temperature super warm and releases sweat so that the body stays warm; the shirt definitely smells like sheep. Also, it's long enough to tuck into pants or undergarments, keeping more surface area of the body warm and compact. You sleep very well at night with this underwear. The first day it itches the body but then you get used to it and it feels like the body has hyperemia in the sense that it is warm. Fully thermal then. I bought two for testing and I will buy more. It's mainly for winter use, and also for heavy autumn.