The Magic Sleepsuit should be used in a room at the recommended temperature for babies, which is on the cooler side. We recommend a diaper, or onesie under the Magic Sleepsuit. We do not recommend any more than one layer of light cotton for breathability. We suggest first trying your baby in the Magic Sleepsuit during a nap, since you can monitor their comfort level. All babies run differently when they sleep. Like any baby sleep product, it is up to parents to monitor the baby in the suit and make sure the baby's room is at the recommended temperature for babies. Parents are responsible to make sure that the baby does not overheat (e.g. your baby should not feel clammy or sweaty to the touch). Please remember that babies' hands and feet can feel cool, so the best way to gauge body temperature is to place your hand on their stomach or back. If they feel hot and sweaty, remove a layer of clothing or remove them from the suit. If they feel cold, add a layer of clothing.