I have a player who's trying to use a sleep effect on a group of mezzoloths in a 2e PBMB combat (think brass dragon breath). It's no middling little 1st-level spell, but something about fiends being susceptible to magically-induced sleep seems... I don't know. Wrong, somehow.
I can't find any ruling on it in PSMC 1. I was wondering if anybody knew one way or the other and could spare me the time it'll take to root through Hellbound and Faces of Evil for an answer.
In Faces of Evil, it specifically mentioned Baatezu as needing at least occasional sleep, and I think the other Fiends do as well. There's a chart in Hellbound detailing Fiend resistances to spells and attacks, in the book Dark of the War I believe.
Yeah, but that's sorta changed in 3x since part of the definition of an Outsider is that they don't require sleep.
That said, anything whose monster stats, type, or subtype lacks any mention of sleep immunity CAN be rendered unconcious/put to sleep.
Ah, thanks! I'd come across the Faces of Evil reference (lesser yugoloths sleep about three hours every three days), but I'd completely forgotten about the immunities chart.
To answer my own question: only a few fiends are specifically listed as being immune to sleep, so I'd have to imagine that the rest are vulnerable.
@Stix - sounds like a reasonable course of logic. 
@Hyena - the OP's game is in 2nd edition, that said - the logic holds for "if it doesn't say they aren't immune - then they are"
In 3e Fiendish Codex 1 it mentions that although fiends (well, Tanar'ri anyway) don't need sleep, they can be rendered unconcious. I assume this means magically, and in rare cases physically.