The best time to recite Seliḥot is the last ashmoret (“watch”) of the night, just before morning, which is a time of mercy and acquiescence, a time of anticipation for the dawn of new light and the revelation of God’s word in the world. At this time, everyone is asleep; the world is pure and unsullied by evil thoughts and deeds. Prayer bursts forth from the depths of the heart, breaks through all barriers, and is accepted on high. Any time after midnight is also appropriate for reciting Seliḥot, as that is when people begin to look forward to dawn; it is also a time of mercy and compassion.
Nowadays, people generally go to sleep relatively late, and wake up between six and seven in the morning, which is generally about two hours after the end of the last ashmoret in Israel. Were they to get up during the ashmoret, they would be tired all day and their work or studies would likely suffer. Therefore, many people wake up a half hour or an hour earlier than their usual time and recite Seliḥot before Shaḥarit. Even though it is past dawn, Seliḥot may still be recited then. Nevertheless, it is better to recite Seliḥot after midnight when possible. In any case, a person should make sure that the recitation of Seliḥot does not leave him so exhausted that he cannot meet all his work or study obligations.
There are those who say that a community that cannot get a minyan for Seliḥot in the morning may, on a temporary basis, recite them before midnight – at 10:00 PM, for instance. (See Igrot Moshe OḤ 2:105.) In practice, however, it is better to say Seliḥot on one’s own at the proper time, since the kabbalists and many poskim say that it is not appropriate to recite Seliḥot before midnight. During the first part of the night, the attribute of justice is dominant, and the world is full of worries and sullied by all sorts of evil thoughts and deeds (Birkei Yosef 581:1-2; Sha’arei Teshuva ad loc. 1; MB 565:12).
There are Seliḥot that refer to waking up at dawn. Some maintain that one who is reciting Seliḥot at a different time must skip these piyutim (AHS 581:4). However, the custom is not to be too exacting about this, because the prayer was instituted for the entire Jewish people, and every day there are some who wake up before dawn to recite Seliḥot.