Keeps Baby warm in the coldest of winter. I put baby in a long sleeve sleeper and zip her up in the Sleep Sack. She sleeps so soundly and I no longer have to worry if shes too cold at night. I call it her baby sleeping bag! She loves it. Its a part of our night time routine. Once she goes in it, she knows its time for bed (and not just a nap). I can rock her right back to sleep in it during middle of the night feedings as well. Used it for 3 months straight. This thing is mom approved!