On Page 101 of the D&D 5e Basic PDF the effect of a Sleep Spell is stated as follows.
This spell sends creatures into a magical slumber. Roll 5d8; the total is how many hit points of creatures this spell can affect. Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points (ignoring unconscious creatures).
Does hit points of creature refer to their current hit points or their maximum hit points.
Maximum hit points are defined as the number of hit point a undamaged creature or character has. Note page 10 of the D&D 5e Basic PDF.
Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum.