The material of the pajamas can be important to consider when shopping for a pair that will help keep you cool in the evenings. “Anything heavy, such as flannel, can trap heat and make it difficult for the natural fluctuation of body temperature during sleep,” Dr. Robbins says. Other important considerations are the “fabric content and construction of the fabric.” Arya says.  “These things can impact what kind of warmth or coolness the fabric can provide. So if it is a knit fabric it can keep the wearer warm or it can keep the wearer cool, also,” she adds. In general, Arya recommends for hot sleepers to stay away from wool and acrylic fabrics if they find themselves feeling warmer in the evenings.