oh, wait... you're just asking for the name of a shirt that clark might wear to bed. that's different. (i'll leave out the fact that most of the time, for some odd reason, clark was shown going to bed without a top...)
there are undershirts. those are just plain white sleeveless shirts. and, even if there isn't something on top of it, it's still called an undershirt. and yeah, there are guys who wear those to bed.
personally, i prefer old tshirts... the kind that have become old favorites, soft and comfortable from repeated washings, but which are no longer suitable to wear outside (a small hole or two, or maybe a stain or something).
there are, of course, actual pajamas. not sure how common they are.
Paul
