Your continued vitality is dependent upon three factors: hydration, nourishment, and rest. If you let any of these factors get too low, and they fall into the red zone, you will be penalised 2 moves each turn they are in the red zone. If any of these factors fall to Zero, you will die.
To keep your rest factor up, you must sleep (in Move menu). Also, if you are in your camp, sleeping will have more effect on your rest factor.