Before one partakes of a meal consisting of bread, he must perform Netilat Yadayim with a Beracha. Are there situations in which one would have to perform Netilat Yadayim again during his meal to be allowed to continue eating?
The Ben Ish Chai (Rabbi Yosef Chaim of Baghdad, 1833-1909), in Parashat Kedoshim (20-23), rules that one must repeat Netilat Yadayim during a meal if he touches parts of the body that are normally covered. For example, if during a meal a person scratches his back, leg or hair, he must wash his hands before he continues eating. He does not, however, recite a Beracha over this washing. If he scratched with only one hand, he is required to wash only that hand.
The Ben Ish Chai further writes that if someone fell asleep during a meal which occurs quite commonly on Friday night and he sleeps for a period of a half-hour or longer, then when he wakes up he must wash his hands before he continues eating. Once again, one does not recite a Beracha when washing his hands in such a case.
Finally, the Ben Ish Chai cites a debate among the authorities as to whether one is required to wash his hands if he prays in the middle of the meal. One common example of this case occurs on Purim, when people often interrupt their Se'uda (meal) to recite Mincha or Arbit. Different views exist as to whether one must wash his hands before resuming his meal after praying. The Ben Ish Chai concludes that if water is readily available, then one should wash his hands, in accordance with the stringent position. If a person in this situation does not have easy access to water, then he may rely on the lenient view and resume his meal without washing his hands.
It should be emphasized that these Halachot are unrelated to the obligation of Mayim Acharonim (literally, "final water"), the obligation to wash one's hands before reciting Birkat Ha'mazon. Our issue involves the requirement to wash one's hands before resuming his meal if he touched certain parts of the body, slept, or prayed in the middle of his meal.
Summary: If during a meal a person touches a part of the body that is normally covered, or sleeps for a period of a half-hour or more, he must wash his hands without a Beracha before resuming his meal. If one prayed in the middle of a meal, then he should preferably wash (without a Beracha) before continuing eating, unless water is not readily available, in which case he may continue eating without washing.
