Hmmm, I don’t know! We only see them in the coffin (for sleeping, as you said, not like in the scene Louis gets locked into a coffin by the TdV) a few times in the film. That could be a sample size problem, maybe other nights they did change into pajamas!

1) There’s the night Louis gets turned, maybe there wasn’t enough time that night to change his clothes but he really ought to have, since his clothes would’ve gotten messy from the procedure (the whole “your body is dying” thing) maybe the filmmakers didn’t want to give him the chance to change clothes bc that would be too caring of Lestat, gotta keep the focus on Lestat being an antagonist.

2) Maybe in these shots w/ Claudia, she is dressed for “bed,” but Louis didn’t have time to change, or maybe he likes dressing in what he’s going to wear the next night… but I think it’s mostly to try to reduce the pedo taboo. He’s a faher figure, even though he’s technically her brother. If he was wearing something more comfortable or showing anymore than face & hand skin, it could be construed as … not fatherly.

IIRC, changing clothes for bed isn’t mentioned in the book!IWTV. I’ll reblog a few posts of vampires in bedclothes for ya ;D

Lestat talks about putting on a special outfit when he stays over at Marius’ house in TVL. He finds a gold mask w/ a hood, leather gloves covered with gold scales, and a blanket with gold plates on one side.

“I realized that if I put on this mask and these gloves – if I laid
over me the blanket – then I would be protected from the light if anyone
opened the lid of the sarcophagus while I slept.”