The sleep spell has an advanced form, castable only at Expert, which creates area of effect sleep like a magic flute does.

Remove sleep as a ray. Sleep wands (and monsters with sleep breath like orange dragons and Nazgul) instead shoot a beam of sleep. The sleep spell either also shoots a beam of sleep or causes sleep in an area of effect. Sleep could also possibly be a cone, spreading out in the direction it’s shot in, but cone spells remain unimplemented.

Make the sleep spell scale like it does in D&D: you can only put a total of (your level) HD of monsters to sleep.

Make the sleep spell scale with your skill level in its school.

Bump the spell of sleep up to level 3 or 4.