High Holidays · Yom Kippur
Who does not fast on Yom Kippur?
Anyone for whom fasting would endanger life does not fast on Yom Kippur. Guarding a life — פִּקּוּחַ נֶפֶשׁ, pikuach nefesh — is itself the mitzvah in that moment, and it sets the fast aside.
Life comes first
Someone who is ill in a way that poses danger does not fast. A woman who is pregnant or who has recently given birth eats as her body and her doctor require. Young children do not fast at all. In each of these cases, eating and drinking is not a leniency granted grudgingly — it is the right and holy thing to do, because the Torah's mitzvot are given to live by them.
The principle and its source
The Talmud in Yoma (82a–85b) teaches the principle of pikuach nefesh: the saving of a life overrides almost every other commandment, Yom Kippur among them. The sages read the verse וָחַי בָּהֶם — "and you shall live by them" — to mean live by the mitzvot, and not be endangered by them. To eat when your life depends on it is to honor the day, not to break it.
What to do
This is a decision to make before Yom Kippur, calmly and with counsel. Speak with your physician about your specific condition, and with your rabbi about how to carry it out — some are guided to eat and drink in small measured amounts (שִׁעוּרִים, shiurim). Bring the question in advance; do not wait until you feel unwell on the day.
Source: The principle of pikuach nefesh; Talmud, Yoma 82a–85b.
So the day can be about your soul rather than the clock, let My Jewish Day (siddur, zmanim, the Hebrew calendar) carry the start and end times of the fast for exactly where you are.
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