Consider fabric. Cotton is breathable and soft, but it may not keep you warm enough. Flannel is warmer, but it may make you too hot in the summer. Silk is expensive and hard to clean. A looser fit is best.
If your feet get cold, which can be bad for sleep, wear socks. But ones that are too thick can heat up your whole body. If you do get too hot, try sleeping naked. It can keep you cool and lead to longer, deeper sleep.