I try many a sleep sack . my little guy love be swaddle and would not go to sleep with out swaddle . but when he get too big for swaddle the thing that keep he warm and comfortable and able to sleep through the night be the baby deedee sleep nest . I get the size medium and my 6 month old boy be pretty big and long , a s big as he 1 year old friend . the sack be roomy and warm . the glow in the dark zipper be a nice touch . the zipper be big and easy to use unlike some I have try . the material be quality and he sleep in it every night , and all through the night . one night my husband forget to put he in it and he wake up all through the night . in the spring I will need to get the next size up . great product .