it is essential you buy the SNOO Sleep Sacks to use with your SNOO. The SNOO rocks your baby to sleep, so they need to be secured in. Your SNOO comes with a set of three SNOO Sleep Sacks in size Small, Medium and Large. We recommend buying a couple of extras as backups when the others are in the wash. For extra warmth and layering, dress your baby in singlets and growsuits under your Sleep Sack.