Sleep Spell???

Sleep ticks up from the lowest health in the area of effect, you don't get to selectively target with it. If everyone in that area has more health than the number you rolled, the spell does nothing except waste the slot spent to cast it.

It's covered in the Sleep spell itself.

A creature's hit points must be equal to or less than the remaining total for that creature to be affected.

Therefore if the target has more hit points than what is left - it simply isn't affected.

If you rolled 99 hp for the spell and there was only one target with 100 hp - nothing would happen. The reminaing hp in that situation would be 99 and the target's hp is not equal to or less than.

If there was a partial effect then the spell would say so. The rules say what happens - if they don't say something they don't do it.

The creature momentarily thinks it needs a nap.