My daugther loved to sleep in her sleep sack all wrapped up nice and tight. We used it until she was about 5 months. It also helped to keep her nice and warm during the cold Minnesota winters! Once she began getting her arms out, we switched to a sleep sack that did not wrap tight around her and now, she only sleeps in her jammies, unless it is a cold night and we need an extra layer. Then, we use the sleep sack to help keep her warm.