The order of the day is divided into two parts: first the morning and then the afternoon, which is called "bain ha-arbayim" [between the evenings]. As discussed earlier, the night is also divided into equal parts. In each of these four divisions, G-d's Illumination and Influence must be transmitted to all universes according to the aspect of that particular time of day.
- From The Way of G-d, R. Moshe Chaim Luzzato, pg 307-309
It is for this reason that a fixed number of daily services were ordained. For the morning and afternoon, there are the Shacharis (morning) and Minchah (afternoon) services. Since the morning is time when G-d's sustenance is renewed for the entire day as a whole, it requires a longer [more inclusive] prayer service. In the afternoon, on the other hand, only the latter portion of the day must be perfected, and therefore a lesser amount of effort is required.
The difference between night and day is greater than that between morning and afternoon, and nightfall therefore involves greater conceptual change. It is for this reason the the Ma'ariv (evening) service is longer than Minchah. The Ma'ariv service also contains the Sh'ma and its blessings, but since the sustenance of the morning service is still retained, the blessings are much shorter.
No universal service was ordained for the second part of the night, since this would unduly burden the community. A midnight service (tikun chatzos) exists, however, but it is only designated for the especially devout, who rise and cry out to G-d, each one accoridng to his own understanding.
Even the evening Amidah itself was not compulsory, but it was later accepted as an obligation (Rif Berachos 84). The Midnight Service (Tikun Chatzos) is all the more so not compulsory. The three main daily service were ordained by the Patriarchs and therefore a universal obligation for every Jew. The Midnight service, on the other hand, which recifies the second half of the night, was something that King David voluntarily accepted upon himself, and he thus sang, (Psalms 119:62) "At midnight I will rise and give thanks to You." Together with the Patriarchs, King David completed the rectification of Israel, as discussed above. Because David was on a somewhat lower level than Abaham, Isaac and Jacob, however, the service that he ordained did not become a universal obligation. It is therefore upheld only by the particularly devout..
On the holy days, a Musaf prayer is added in lieu of the Temple Musaf Sacrifice that was offered on those days. Tjis is related to the additional sustenance that is granted on such days according to their holiness and other aspects.