The Be’er Heteb commentary to the Shulhan Aruch (Orah Haim 238:2) cites the ruling of the Arizal (Rabbi Yishak Luria of Safed, 1534-1572), in Sha’ar Ha’misvot (Parashat Vaethanan), that it is proper to read Tanach only during the day, and not in the nighttime hours. This ruling, which is based upon concepts of Kabbalah, is also cited and accepted by other authorities, including the Hid"a (Rav Haim Yosef David Azulai, 1724-1806) and the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909). Accordingly, the Ben Ish Hai writes (as does the Kaf Ha’haim), one should not recite Tehillim during the night.
Hacham Ovadia Yosef addresses this topic in his work Yabia Omer, and rules that it is indeed proper to follow the Arizal’s practice and refrain from reading verses of Tanach during the night. He adds, on the basis of the Hid"a’s ruling, that this applies throughout the entire night, until morning.
However, Hacham Ovadia makes an exception in this regard for cases of a seriously ill patient, Heaven forbid, when friends and relatives wish to recite Tehillim to pray for his or her recovery. In such a case, Hacham Ovadia rules, it is permissible to recite Tehillim on the patient’s behalf after Hasot (midnight as defined by Halacha, which occurs, depending on the season, anywhere from 11:30pm-1am). Tehillim should not be recited before Hasot, even in situations of an ill patient, Heaven forbid. Instead, people should study material such as Mishnayot, Halacha or Gemara in the patient’s merit until Hasot, and then recite Tehillim after Hasot.
There are several other exceptions to this Halacha, as well. It does not apply on Thursday night, Friday night, or the night of Yom Tob. The Ben Ish Hai writes that the night of Rosh Hodesh is considered an ordinary weeknight with respect to this Halacha, and hence one should not read Tanach on the night of Rosh Hodesh.
Furthermore, this Halacha applies only to reading. It is permissible to study Tanach at night, meaning, learning the text with at least the commentary of Rashi. The prohibition applies only to simply reading the words; studying the material with commentaries is allowed at nighttime.
Additionally, this Halacha applies only to those who are able to study other material, such as Talmud and Halacha. If somebody does not have a background in other subjects, and the alternative to reading Tanach is engaging in unproductive activities, then it is certainly preferable for him to read Tanach at night rather than waste his time. In such a case, the Ben Ish Hai advises the individual to study parts of the Oral Law that he knows from the prayer service, such as Patah Eliyahu and Ezeheu Mekoman. But if his only alternative to reading Tanach is wasting time, then he should certainly read Tanach.
Summary: It is proper not to read Tehillim or other parts of Tanach during the nighttime hours, except on Thursday night, Friday night, and the night of Yom Tob. This applies only to reading; one may study Tanach with commentaries at night. If a person is incapable of studying other material, and the alternative to reading Tanach is wasting time, then he should read Tanach at night rather than be unproductive. In cases of an ill patient in need of Tehillim, Heaven forbid, people may recite Tehillim on his behalf after Hasot (midnight as defined by Halacha).
