I wear this anytime I need long underwear. I don't even touch "separates" anymore. The waist zip design makes going to the bathroom just as convenient as two pieces, but the one-piece design of the Ninja Suit is superior in every way. I own an older synthetic version of the Ninja Suit, but the Merino is definitely the go-to unit. Merino rules. I'm 5'8" 175lbs and a Medium is the right size for me. I could probably wear a small, but it would be pretty tight.