How do I use the bed?

[Dev]

I bombed into the "inn", and I went under the rug to find an item. I recall hearing that you can sleep in the bed. How exactly do I do this? I can't just run into it. Do I need bombs or keys or money or something? Is sleeping in the bed even worth it? I get the feeling all it does is restore your health, so I may not need it. I hope for a quick response, and thank you.

[Dev]

Okay, I walked into the bed, but this time after beating the boss and holding a bomb and key - and I awoke to find three more soul hearts than I came in with.

[Kanvas]

The bed acts as a full heal if you have taken red heart damage
It will give you 3 soul hearts if you have no red heart containers.

Run into the bed to use it
If you have all full red heart containers, it will not do anything until you take damage.

[Apocyl]

If you use ???/Blue Baby it always gives you 3 Soul Hearts (except you got it full and the last 3 hearts are black hearts)