Our 1.0 Tog Jersey Sleep Sack now with sleeves is a year-round sleep sack for an infant or toddler sleeping with their arms-out usually from around 3 months of age. The Jersey Sack has a slim-fit, stretchy design similar to that of a swaddle and is recommended as baby's 'first sleep sack' after the swaddling phase is over. The ergonomically designed sack features a bell-shaped bottom to promote healthy hip joint development and is approved by the International Hip Dysplasia Institute, and the material is stretchy to allow baby to move more freely and stay comfortable while they sleep. The added sleeves provide a little more warmth on cooler nights, or for those whose body temperature naturally runs cooler.