I've been put to sleep by a Sloom in addition to being creepily beaten. I have a dirty mind.
While the sleep counter is going down, the Sloom will attack me, sometimes wake me up, and... heal me for about 2 bars of health.
It's nice that the Sloom will wake me up once in a while but... heal me at the same time? Is this intentional?
The Sloom in particular is the smallest sleep sloom without the spikes.
I've noticed when using sleep vials that the enemies too will gain health if you let them rest for a while. Same deal, I believe.
This is also why I stopped bothering with sleep vials.