There's been some discussion lately about how low-level MUs are extraordinarily dangerous to low-level parties; just as sleep wipes a beastman gang, so too does sleep wipe a group of first or second level PCs. Even adding a save doesn't help that much, because low level characters have terrible saves. So the discussion has mostly turned to initiative and disrupting casters.
There was another discussion about how magic missile is really useful for this, to the point that there exists a spell designed primarily to counter it (shield), with accompanying magic items (brooches of shielding).
If sleep is dangerous enough that high-level wizards keep losing their best apprentices to other schools who cast sleep, maybe they'd solve this problem with magic, in the same way.
So kicking around the Player's Companion's rules for protection spells, here is a spell that might be useful for low-level characters expecting to go up against wizards:
Duration: 12 turns
Creatures within a 7.5' radius of the caster are immune to the spell sleep for the duration.
(Points: immunity to sleep 5, range self x0.75, 7.5' radius x1.5, duration 12 turns x1.75 = 9.8)
There's a little room here to fiddle with the duration/radius tradeoff. 7.5' radius won't cover the whole party, but a good chunk will still be standing. The other edge of the sword, though, is that enemies who are up in your face are also immune to sleep (which is very consistent with my feeling that arcane magic should over-solve problems).
Another fun avenue, of course, is potions.
Potion of Wakefulness
This dark brown, bitter potion prevents the drinker from falling asleep by any means, mundane or magical, for six hours. For the first hour after drinking it, the imbiber gains a +1 bonus to surprise rolls due to hypervigilance, but takes a -2 penalty to attack throws due to jitters. Characters who use Potions of Wakefulness more than once in any given week halve the duration for each potion after the first (second potion lasts 3 turns and 3 hours, third potion lasts 1.5 turns and 1.5 hours, etc).
A final option for dealing with sleep in NPC hands might be the friendly fireball approach. OD&D's text for sleep is predictably ambiguous about targeting, but "The spell always affects up to the number of creatures determined by the dice" could certainly be read to mean that once it has put the monsters to sleep, if it has any HD left over, it tries to put the party to sleep. Changing ACKS' sleep to operate in this way moves it from "a spell to solve almost any non-undead combat encounter on the first two dungeon levels" to something you have to be a little careful with, because if you use it on too small a group and you roll high you could put your entire party to sleep, and then you're probably toast. And if you want to have multiple henchwizards all casting sleep in the same round, so that any one disruption doesn't stop it from going off, then you have to worry about what happens if none of them get disrupted.