Sleep effects a number of creatures according to their hitpoint totals. The duplicates produced by Mirror Image are not creatures and do not have hitpoints.
Ergo, the target gains no benefit from Mirror Image.
Edit: Sleep is an AOE, so it targets an area, not a specific individual.
A wizard casts Sleep on a target that is using the Mirror Image spell. How would you rule on this being handled?
Sleep effects a number of creatures according to their hitpoint totals. The duplicates produced by Mirror Image are not creatures and do not have hitpoints.
Ergo, the target gains no benefit from Mirror Image.
Edit: Sleep is an AOE, so it targets an area, not a specific individual.
I think memnosyne covered everything.
Tl;dr: Sleep works the same as if mirror image was not active.