I went along doing the one-salad-a-night routine for a year. And I remember feeling so tired and depressed and irritable. I had no personal life. I was always flying someplace - weekends, holidays, vacations. Dinners at night were no fun because I couldn't eat.
Usually you talk about directors in terms of the way they choose camera lenses or a kind of light to create a certain effect. But to me the most valuable commodity for a movie to create is a feeling of life, and that's what A Hard Day's Night has in spades.