Hello, is it permitted for a woman to wear a man’s pajama pants and a mans undershirt to sleep?
If the pants and undershirt are clearly menswear, they should not be worn by a woman. Many “training pants” are “unisex,” and they would be permitted to wear.
Sources: See Shulchan Aruch, Yoreh De’ah 182:5; it is possible that the prohibition would not apply to underwear, which cannot be seen. See Beis Yosef 182:5, and Bach, who writes that the prohibition applies specifically to clothes that are a kishut, meaning worn for show. However, the prohibition probably apply to pajamas, which are styled especially for men, and made to look stylish.