There are three degrees of eating: self-control, sufficiency and satiety. Self-control is to be hungry after having eaten. Sufficiency is to be neither hungry nor weighed down. Satiety is to be slightly weighed down. To eat again after reaching the point of satiety is to open the door of gluttony, through which unchastity comes in. Attentive to these distinctions, choose what is best for you according to your powers, not overstepping the limits. For according to St Paul only the perfect can be both hungry and full, and at the same time be strong in all things (cf. Philippians 4:12).
~St Gregory of Sinai
Prayerful Life 