Much of the issue of the timing of the minhag to recite Selichot in the days before Rosh Hashanah and Yom Kippur is based on kabbalistic considerations, which are not within our area of expertise. However, we can discuss the basic ideas as filtered through the poskim.
Different times of the day have different characteristics, making them more or less appropriate for certain types of religious activity. The first part of the night possesses the characteristic of din.[2] The second part of the night is an eit ratzon.[3] Thus, the latter is the time when the Selichot prayers are most appropriate, as they are specially formulated to elicit mercy from HaShem.
Several classical sources[4] and several of the piyutim[5] themselves refer to Selichot being said at ashmoret haboker, during the few hours leading up to alot hashachar, which itself is around 72 minutes before sunrise. At this time in particular, HaShem hovers over our world,[6] making it an eit ratzon. There are also sources that indicate that chatzot[7] is a special eit ratzon.[8] Therefore, the optimal times to say Selichot are either after chatzot or in the predawn hours. Rav Moshe Feinstein[9] points out that in previous generations, people generally went to sleep and woke up earlier than most do today, and the predawn option was therefore more convenient and common. However, he continues, one should not infer a clear preference between these times from that phenomenon. Only on the first night of Selichot (for Ashkenazim), on Motzaei Shabbat, there may be a preference to say Selichot at chatzot, when more of Shabbat’s impact remains.[10]
The main objection is to saying Selichot at night before chatzot, a time of din. Although Selichot have a special power to elicit mercy – especially the Yud Gimmel Middot,[11] which is their most basic component – kabbalistic sources say that it is spiritually dangerous to recite them at a time of din, since it is viewed as though mercy is overstepping its bounds.[12] Rav Feinstein is identified with the camp that does not put a strong emphasis on kabbalistic sources in making halachic decisions. He rules[13] that even though much positive effect is missing when Selichot are recited during the first half of the night, if a feasible alternative is lacking, it is better to recite them at that time than to deprive the congregation of its inspiration in preparing for the Yamim Nora’im.[14] Rav Feinstein prefers reciting Selichot at a change of ashmorot, one of which is approximately two hours before chatzot. However, other poskim counsel to avoid the strongly detrimental situation that the kabbalists describe even when there is no easy alternative.[15] Some suggest that early night in America is not so bad because it is after chatzot in Israel. However, we attribute the times of din and eit ratzon to each place according to its astronomical situation.[16]
The safer approach for those who find it impossible or at least difficult to recite Selichot after chatzot or before dawn is to do so in the morning.[17] While it is not especially an eit ratzon, it is not a time of din either, and the period of the year is itself an eit ratzon.[18] For many people, this is much more manageable and does not sap as much energy that could be used for other mitzvot. The general approach is that it is worthwhile to sacrifice a modest amount of quality and quantity of Torah learning in order to say Selichot.[19] However, one has to make the difficult evaluation of whether he is capable of fulfilling his daily responsibilities while dedicating some of his physical resources to reciting Selichot at the optimal time.