Henleys are in between a T shirt and a sweater, medium thickness longsleeved, no collar but with a couple buttons at the top It's a better casual option than a T shirt, especially if it's slightly chilly and they can be easily layered with jackets, flannels or sweaters over them. I pair them with a short sleeved collared dress shirt often, good for those spring/fall days where the temperature shifts signifiicantly throughout the day. They also make comfortable sleeping clothes that work well with sweatpants.