Usually we put little one to bed in a long sleeve onesie and a fleece sleep sack over top of that.  However with these PJs we put a tank top (sleeveless) onesie on UNDER and the PJs over it.  And no sleepsack,  They are very thick and high quality fleece.  Love these!  Order a size bigger than you think.  Little one is currently in size 8 carters, but a size 12 in these gerber PJs.