The Magic Sleepsuit should be used in a room at the AAP 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.
Like with any baby sleep product, it is up to parents to monitor the baby in the Sleepsuit and make sure the baby's room is at the recommended temperature for babies. 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 Sleepsuit.