The time to recite Shema in the morning is learned from the expression that appears in the Torah passage regarded the Shema, “u’vekumecha,” “when you get up,” meaning the time when people normally wake up. There are people who rise early, from the time of amud hashachar (approximately 72 minutes before netz),1 therefore, biblically, it is considered the time of waking and from then on one may recite Keriat Shema. The reason for this is that the words “when you get up” are written in the singular person, implying that even when only a few people arise from their sleep it is time to recite Keriat Shema. One who recites Keriat Shema before then does not fulfill his obligation, since his recital precedes the time of waking.
The Chachamim “created a fence” (asu siyag) to the mitzvah and established not to recite Keriat Shema l’chatchilah until the time when more people usually arise from their sleep, at which point there is already more light on the earth and one can recognize his friend from a distance of four amot (Shulchan Aruch 58:1). This is the time of misheyakir (approximately 50 minutes before netz in the days of Nisan).2
When a person cannot recite Keriat Shema after the time of misheyakir due to circumstances beyond his control, he may recite it from amud hashachar. Similarly, b’dieved, if one mistakenly recites Shema from the time of amud hashachar, he fulfills his obligation. However, there is a difference between these two cases. If one’s reason for reciting Shema early emanates from circumstances beyond his control, even if he must do so every day, he fulfills his obligation. But if he recites it early due to error, he fulfills his obligation only if his mistake seldom occurs, meaning not more than once a month. If he errs more often than that, the Chachamim penalize him; he has not fulfilled his obligation and he must go back and recite it again after misheyakir (Shulchan Aruch 58:3-4; Mishnah Berurah 58:19).
The time to recite Keriat Shema lasts until the conclusion of the first three hours of the day, for there are people, such as princes, who continue to sleep until the end of the first three hours. Therefore, until the end of the first three hours is still considered to be the time of “when you get up” (this is clarified further in halachot 10 and 11).
The most praiseworthy time to recite Keriat Shema is at vatikin, meaning slightly before netz hachamah.