Graveyard Keeper has only one way of saving - you have to get up from the bed. You don't have to get a full sleep, simply go to bed and then get up immediately. This is the only way in which you can save the game so remember about that. The game won't save automatically - you need to go to bed at least once a day.
This is problematic in the later parts of the game, when the majority of your energy is restored with meals and potions.