Is one allowed to say Tehillim at night?
I never received a clear answer from anyone about it…
Would you also be able to provide an explanation of why one would not be able to? (I heard s/t about the night being time of din…however, wouldn’t that be an ideal time too counter that with Tehillim that evokes rachamim?)
For times of need, it is permitted to recite Tehillim during the entire night — at least for Ashkenazim. When there is no specific need, some maintain that it is nonetheless permitted, whereas other authorities, who heed kabbalistic sources, are more stringent. In the second half of the night (after chatzos) all are lenient with regard to reciting Tehillim.
Sources: Kabbalists write that the night, and in particular the time of night between nightfall and chatzos, is not a suitable time for study of the Written Torah (verses of Scripture), which includes the recitation of Tehillim. See Chaim Sha’al 2:25; Kaf Hachaim 228, and many other sources, all based on Arizal. It is unclear whether or not Arizal intended this prohibition to apply through to nightfall, or only until chatzos, but the common custom is to recite Tehillim after chatzos (see Yabia Omer, vol. 6, no. 30:6), though the study of verses other than Tehillim is reserved for the actual time of day.
As you correctly note, the reason given for this by Kabbalists is that the time is a time of din (strict judgment). This is a deep matter of kabbalah, but the basic idea is that Torah verses are also related to din (as indicated by their general vagueness and the need for elucidation of the Oral Law), and it is therefore considered wrong to awaken dinim by reading verses at night. See Shaar Hamitzvos (of Arizal, Parashas Va’eschanan); Yosef Ometz (of Chida, no. 54); and see Birkei Yosef (238:2) who brings a source from the Midrash concerning the distinction between night and day: day is a time for the Written Law, and night for the Oral Law.
When there is a danger to somebody’s life, it is customary to ignore the kabbalistic prohibition of studying verses at night, and to recite Tehillim even during the first half of the night.
Moreover, Tzitz Eliezer (8:2, 17:3) writes that for times of need it is permitted to recite Tehillim at night, and it is customary to recite Tehillim on the night of Hoshanah Rabbah (Betzel Hachochmah 4:46). Mishnah Berurah (Shaar Hatzion 238:1) cites a dispute over the study of Scripture at night, and writes that there is certainly no prohibition, but only that it is better to study other parts of the Torah (the Oral Law) at night. This implies that wherever there is any need, it would be proper to recite Tehillim at night.