Eating prior to Alos:[1]
It is forbidden to begin a meal starting from 30 minutes prior to Alos [however, snacking is allowed]. [Nevertheless, some Poskim[2] rule it is a grave prohibition according to Kabala to eat, prior to dawn, after awakening, throughout the entire year. This prohibition applies starting from midnight and onwards, after awakening from a standard sleep of 60 breaths.[3] Practically, the custom of Sefaradi Jewry is to be stringent.[4] However, Ashkenazi Jewry is lenient[5] and so is the Chabad custom.[6] If one did not sleep that night for at least 60 breaths, then according to all, he may eat up until dawn.[7] Likewise, some Poskim[8] rule that if one plans to return to sleep for a second time before Alos, then this prohibition does not apply. Likewise, if one is weak or sick he may eat prior to Alos even according to the Zohar.[9]]
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[1] Admur 89/5; Rama 581/2
[2] M”A 89/14 in name of Tov Haaretz, Rav Chaim Vital and Zohar 2/215; Beir Heiytiv 581/12 in name of Zohar and Kisvei Arizal; Chida in Machazik Bracha 581/5; Birkeiy Yosef 89/1 [Shiyurei Bracha] in name of Mekubbalim; Yosef Ometz 17/2; Mishnes Chassidim Chatzos 7; Oar Hachama Zohar ibid; Nitzutzei Oros Zohar ibid; Kaf Hachaim 581/69; 89/28-31]
The reason: As one who eats food past midnight after awakening adds vitality to the Kelipos. [Rebbe ibid] It is however not prohibited due to the prohibition of eating prior to prayer, as this prohibition only begins from Alos. [89/5; Rebbe ibid]
Other Poskim: Some rule that even according to Kabala there is no need to refrain from eating prior to Alos. [Heishiv Moshe 6 based on Siddur Arizal of Rav Shabsi of Rashkov that Rav chaim Vital ate before Alos; Mahril and Rama ibid]
Opinion of Shulchan Aruch Harav: In Igeres Hateshuvah chapter 3 Admur writes that one may eat prior to three hours before sunrise on a fastday. The Rebbe in Igros Hakodesh 3/75 writes that this ruling only was said regarding when the prohibition of eating prior to a fast begins and is not discussing the prohibition of the Zohar, which in certain cases is allowed. Nevertheless a proof can be brought from Admur 89/5, which omits this law of the M”A ibid brought from the Zohar, that Admur rules it is permitted.
[3] So is evident from M”A ibid; Beir Heiytiv ibid; and other Poskim; Igros Hakodesh 3/75
[4] Kaf Hachaim ibid
[5] Shvus Yaakov 3/41 defends the ruling of Rama; Some Poskim write that the current Ashkenazi custom is to only be lenient to allow to drink coffee or tea prior to Alos, although regarding eating food they are careful. [Shaareiy Teshuvah 581/12; M”B 581/21]
[6] See Igros Hakodesh 3/75 [brought in Shulchan Menachem 1/44] that so is the ruling of Admur. It is our custom to allow one to eat after Alos before Davening in order to have strength to Daven. Certainly then, one may be lenient to eat before Alos in order to have strength to Daven.
[7] P”M 89 A”A 14; Igros Hakodesh 3/75
[8] Ashel Avraham Butchach [Tinyana] 89; See there that he connects the prohibition of the Zohar to the prohibition of eating before Davening.
[9] Nitzutzei Oros Zohar ibid; A”A 89 in name of Shev Yaakov [brought in Igros Hakodesh ibid]
According to the stringent opinion, may one nevertheless be lenient in a case that there is a mitzvah for him to eat, such as one who awoke from his sleep right before alos and realized that he did not yet eat matzah on the first night of Pesach, or one needs to eat melava malka?