Press R to rest. Every 25 seconds you will recover 1/2 HP. You may only rest when you only have half HP. Resting recovers max half HP. Good idea or not? Won't make mushrooms useless (quick way to gain more than half HP), and maybe when resting you take twice the damage if you have been hit.
Half a heart. If you make sure mobs arent around, you should be pretty save. Through you should not me able to move/look around while resting. And I think triple damage would be a better idea than just double damage.