840. A Last-Minute Maariv
It is a mitzvas aseh (a positive Biblical commandment) to recite the Shema both in the evening and in the morning. The ideal time for saying the evening Shema is during Ma’ariv (the evening prayer); Shema and Ma'ariv should be recited immediately after tzes hakochavim (nightfall, literally, when stars emerge)—halachically defined as the time when three kochavim ketanim (minor stars) are visible to the naked eye in the night sky.
A person who is delayed from reciting the evening Shema at the optimal time may still do so until chatzos halaylah (midnight; the sixth halachic hour after sunset). This deadline was established l’harchik adam min ha’aveirah (to spare a person from transgressing), to avoid a cavalier attitude toward the obligation to recite the Shema in its correct time.
However, b’dieved (post facto—i.e., chatzos has passed) a person may recite the Shema until alos hashachar (dawn, “the rising of the morning [sun’s rays]”), after which the time for reciting Ma’ariv and the Shema has passed. Someone who is an anus (“forced”) due to unavoidable circumstances—for example, one who is too drunk to daven before alos hashachar—should nevertheless say the Shema and Ma’ariv until netz hachamah (sunrise—when the sun is visible on the horizon). In this case, the tefillah (prayer) of Hashkivenu (“Put us to sleep…”) should not be said, since it is limited to the time when people normally go to sleep. There is also a machlokes haposkim (a halachic dispute) whether Shemoneh Esreh (the nineteen blessings [of the silent Amidah]) of Ma’ariv may be recited at this time.