The first time of the day that has halachic significance is known as “amud hashachar” or “alot hashachar” (dawn). In a case of extenuating circumstances, one may begin reciting Shema and praying the Amidah from the time of amud hashachar, but under normal conditions it is prohibited. The Acharonim disagree whether amud hashachar is the time when the first glimmer of light appears in the east (Magen Avraham, Pri Megadim), or slightly afterwards, when that first light spreads out somewhat upon the eastern sky (Eliyah Rabbah, Gra).
After amud hashachar, the time called “misheyakir” arrives. This is when enough light has already spread over the earth so that a person can recognize a friend, whom he is not accustomed to meeting, from a distance of four amot. At that time it is also possible to distinguish between techelet (a shade of skylike blue) and white. According to most poskim, l’chatchilah one may recite Keriat Shema then, but the time to recite the Amidah l’chatchilah has not yet arrived.
“Netz hachamah” is sunrise, when the first part of the sun is visible. At that moment, the time to recite the Amidah l’chatchilah begins. One who recites Keriat Shema right before sunrise and prays the Amidah immediately with netz hachamah is known to be praying “vatikin,” greatly praised by the Chachamim.
All of the mitzvot performed during the day, such as brit milah, begin at sunrise, since the time of day is defined by the sun. However, b’dieved, if they are performed from amud hashachar their obligations are fulfilled because, from a certain standpoint, the day begins even from the first glimmer of light (Megillah 20a).