This all depends on the temperature of your nursery. Typically, a cotton onesie works well; if your sleep sack is sleeveless go for a long-sleeved onesie. Different climates and surrounding will have a great bearing on what your child should wear too. Those in colder climates will have to dress warmer than those in more sunnier areas. Just use your best judgment and watch the infant closely for fussiness or overheating signs. They will generally let you know if they are hot, uncomfortable, or unhappy.