Not quite. Think of it more like snakes or other cold-blooded reptiles, except with mana instead of heat. Mana level goes down, they're weaker, sluggish, and hibernate. It starts rising, they wake up, and their powers start coming back. For food, the only example I've seen of a dragon actually discussing their eating habits in any detail is from the Earthdawn book Dragons, where Vasdenjas was talking to his scribe, got hungry, and went out for a bite, bringing back a whole cow. Vasdenjas said that he doesn't like to eat his fill in one sitting, because it makes him sluggish, so he tries to break things up and have a few meals throughout the day.