Your baby’s comfort level is the best indicator of what he or she should wear under Baby Merlin's Magic Dream Sack. All babies run differently when they sleep. We suggest putting your child to bed in whatever makes them comfortable - a onesie, lightweight cotton pajamas, etc. Like any baby sleep product, it is up to parents to monitor their baby's comfort level 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. If they feel cold, add a light layer of clothing.