The prohibition of eating and drinking before prayer begins at amud hashachar, for that is the earliest time one may recite Shacharit. Regarding eating a meal the prohibition begins a half-hour before amud hashachar, lest he become so involved in his meal that he will forget to recite Keriat Shema and the Amidah. However, eating a snack is permitted before amud hashachar. Therefore, it is permitted to eat an unrestricted amount of fruits, vegetables, and cooked food before amud hashachar. Even eating an unlimited number of cooked foods made from various types of grain, such as pasta, is permissible before amud hashachar. However, bread and cake are permissible to eat only in an amount less than k’beitzah (like an egg), for that quantity does not constitute the eating of a meal (Shulchan Aruch 232:3; Mishnah Berurah 35; Sha’ar HaTzion 89:33).
Prior to the half hour before amud hashachar, one is permitted to eat anything. Nevertheless, once amud hashachar arrives, all eating and drinking must cease (Shulchan Aruch 89:5; Mishnah Berurah 27 and 29).
According to Kabbalah, some are accustomed to act stringently, maintaining that anyone who awakens after a regular sleep at night, even before chatzot, must not eat and drink until after praying Shacharit. Even though according to halachah it is permitted to eat and drink before amud hashachar, l’chatchilah, it is proper to be cautious in doing so. However, if the lack of food will cause neglect of Torah learning, it is better to eat and drink before amud hashachar (Mishnah Berurah 89:28). Similarly, those accustomed to waking up on Shabbat night to recite special prayers (“bakashot”) are allowed to eat and drink, especially if that will help awaken them more to serve Hashem. (Those who follow Kabbalah are more stringent concerning this. See Kaf HaChaim 89:28 and 43; see Yabia Omer, part 5, 22:5-6).