The polymorph spell mentions that the creature assumes the new form's hp. As long as that hp doesn't hit 0 hp, the creature doesn't revert. But it doesn't state that you can't polymorph a creature at 0 hp.
As such, could you technically polymorph an unconscious creature to prematurely wake it up?
From the Polymorph spell description:
This spell can't affect a target that has 0 hit points.
Of note: this caveat was added in the Player's Handbook Errata, as pointed out by Thyzer, after the original printing. Some PHBs may not contain this text, but it nonetheless is the intended rules.
Other spells with polymorphing effects don't work either:
True Polymorph says
Choose one creature with at least 1 hit point
and Mass Polymorph adds temporary hit points for the new form, but since your base hit points are still at 0, you'll remain unconscious.
