Tried it out now. Never played any minetest before, so I'm mainly confused about it not being minecraft! Laptop's dead now so I'll figure out why the sleeping spot isn't giving me any energy later!
The sleeping spot is terrible, it's basically just marking out a spot and laying on the cold, hard ground. Useful when you've fallen in a hole and can't get out, but if you've got access to grassy plants, make a sleeping mat from eight of them, it's much better.
Also to regain energy you'll need to be comfortable, if the surrounding temperature is too cold or hot it can prevent resting. Get a fire and a roof of some kind -- a shallow cave or tree works, though trees lose their leaves in winter which makes them less useful.
We have a tutorial in the works to help newbies with these things, but it's not ready yet.