Halakhah sa Mga Awit 55:18: Mishneh Torah, Prayer and the Priestly Blessing & The Sabbath Epistle

So also they ordained that a person should recite one service of Prayer at night, since the portions of the animal offered up as the Afternoon Sacrifice were consumed on the altar throughout the night, as it is said, "This is the law of the burnt offering; this it is that which goeth up on its firewood upon the altar all night" (Leviticus 6:2). And in this sense it is said, "Evening and morning and at noon will I meditate and cry aloud, and He hath heard my voice" (Psalms 55:18). The Evening Service is not obligatory like the Morning and Afternoon Services. Nevertheless all Israelites, wherever they have settled, have adopted the practice of reciting the Evening Service and have accepted it as obligatory.

Thus the poet (David) said, “Evening, morning, and noon” (Psalms 55:18), and he did not say “morning, noon, and evening”. He mentioned the three times when a person is obliged to pray, for the other(?) time people are asleep. That is “At midnight I arise to praise You” (ibid. 119:62).