How to prevent dominance of sleep at low levels?

[+1 Dagger]

You can also house-rule that spell-casters may not use their "bonus" spells to double (or triple) up on Sleep spells; i.e. those bonus slots must be used for different spells.

[Longman]

Another issue is how the caster got the spell in the first place.

The rules say the caster begins with two spells (read magic and one other) and then have to find and pay for any new spells.

Did you (the OP) agree to give 'Sleep' as the 1st level spell or did you actually roll an 11 randomly on the spell table?

Sleep and Charm Person are the gems of the 1st level table. Web, ESP and Invisibility are the 2nd level equivalents. Part of the fun is making such spells hard to come by. Or at least, ramping up the threat level if you allow free choice.

Otherwise allowing free choice is like giving every fighter free plate mail.

[Solomoriah]

But plate mail is a lot cheaper than a spell... most fighters can afford it after their first good haul.

[Longman]

OK, it was a limited analogy. All I mean is, free choice of spells is a pretty big advantage.