Blood Suger Levels.
Way back in the days when B52s did twenty four hour airborne patrols the USAF did a study of crew and produced a fat paper on their results.
They studied pre, in and post flight rest, eating habits and suggestions and a few other things. One of their findings was that, to maintain adequate blood sugar levels, one should have a high carbohydrate snack every thirty to sixty minutes!
(They also discovered that by keeping the cockpit dome lights on during most of the cruise it was possible to reduce fatigue by up to 70% - but that is a different subject!).