The custom to recite Ma’ariv immediately following Minchah arose from the concern of some rabbis that some individuals who attended the Minchah prayer would not return to the synagogue after dark to recite Ma’ariv and would perhaps forget to recite the Shema in their homes altogether. Therefore, it would be better to recite the Shema early than not at all.
According to Tosafot, it is perfectly proper to recite the Shema in Ma’ariv immediately after Minchah even though it is still light. According to Rashi, even though it is appropriate to recite the Shema in Ma’ariv while it is still light, one should repeat at least the first paragraph at home when it is dark. For this purpose, it is sufficient to recite the first paragraph of the Shema before going to sleep as part of the bedtime prayers. According to the Mishneh Berurah, however, one must recite all three paragraphs of the Shema at home after dark.
Although according to the Torah the nighttime Shema may be recited until dawn because some people go to bed after midnight, the rabbis decreed that the evening Shema should be recited before midnight so as to avoid a situation where people will fall asleep and miss reciting the Shema.
In cases of emergency, however, the rabbis permit the recital of the Shema until dawn. In situations of dire emergency, such as when a person is sick, the evening Shema may even be recited after dawn and before sunrise provided it is not accompanied by the blessing of Hashkiveinu.
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