There is a fundamental difference between the times set for reciting the Shema and all other prayers. Whereas our sages linked the times for prayers to the times of the Temple sacrifices, the time for reciting Shema is fixed by the Torah itself – “beshochbechah uvekumechah” – when you lie down and when you get up.
Accordingly, the morning Shema is to be recited when people usually get up. The earliest time the Shema can be recited is just after Amud Hashachar, which is the time the first ray of light begins to shine on the Eastern horizon. Amud Hashachar itself would be too early because it is still dark and one would not be able to fulfill the requirement of seeing the tzitzit when reciting the third paragraph of the Shema.
The preferred time for reciting Shema in the morning is just before hanetz hachamah,” sunrise. Hanetz hachamah is when the sun begins to shine from the mountaintops and it is about one and one-fifth of an hour after Amud Hashachar. This is the preferred time because it is especially meritorious to recite the Shema immediately before reciting the Amidah prayer. But since the Amidah prayer cannot be recited before sunrise, the only way to avoid a break between Shema and the Amidah is to recite the Shema immediately before sunrise and the Amidah immediately thereafter.
In case of emergency, such as when one has to leave on a journey right after dawn and will not be able to recite the Shema until beyond its latest permissible time, one may recite Shema at the crack of dawn because there are at least some people who get up at that early hour.
The latest time Shema may be recited in the morning is the time by which most people have already risen. That time is set by the sages to be when the first quarter of the day is over – which is the end of the third halachic hour of the day. A halachic hour is the unit of time derived by dividing the period of time between sunrise and sunset by twelve. It can be more or less than sixty minutes depending on the time of the year. Thus, if there were eighteen hours between sunrise and sunset on a summer’s day, the latest time for reciting the Shema would be four-and-a-half hours after sunrise.
In the summer, when the sun rises early, that would occur earlier than one might think. If there were only nine hours between sunrise and sunset on a winter’s day, the latest time for reciting the Shema would be two-and-a-quarter hours after sunrise. Taking into account that the sun rises earlier in summer than in winter, the latest time for reciting Shema occurs at an earlier hour in the summer than it does in the winter.
Accordingly, whereas during the month of December the latest time for reciting the Shema is 9:03 a.m., in June the latest time is 8:37 a.m. In view of the fact that the times for reciting the Shema are fixed by the Torah itself, particular care must be taken to recite it before the first quarter of the day.
The timely recital of the Shema is so important that if one cannot reach the synagogue or access ones tallit or tefillin before the first quarter of the day, one must recite the Shema in private or without tallit and tefillin and then repeat it later on in the synagogue or when tallit and tefillin are at hand. Particular care should be taken to recite the Shema at home if one attends a synagogue that begins late on Shabbat morning.
The evening Shema is to be recited at the time people go to bed. Accordingly, the earliest time for reciting the evening Shema is when three stars are visible in the sky or would be visible were the sky not obscured by clouds.
There is much discussion among halachic authorities on the proprietary of the custom to recite the evening Shema during a Ma’ariv service held immediately following the recital of Minchah one and a quarter hours before night, a time known as Plug Haminchah. The problem is that at that time there is still light and it is difficult to call it nighttime because people do not go to bed while it is still light outside.