Time: Reciting Tehillim at night
1) The Mishne Bruro (Shaar Hatzion §238) brings a machlokes between the באר היטב and מגן אברהם whether it is permitted to read 'מקרא' at night. The MB writes that even according to those who say not to, it isn't prohibited, just that it is preferable to say it during the day. (Those who don't allow it are based on the Medrash [Pirkei d'Rebbi Eliezer 46 and Tanchumah Parsha Ki Sisa 36] that when Moshe went to heaven for 40 days/nights to receive the Torah, he was taught Torah Shebichsav by day only.)
2) The Tzitz Eliezer (Ch. 8 §2) brings a list of Poskim who write that this prohibition includes only חמשה חומשי תורה, not נ״ך. Accordingly, tehillim would be permitted.
3) Even according to those who hold that נ״ך is included in the prohibition, it could be ספר תהילים is excluded. (The Chida [Yosef Ometz §54] brings from a מקובל מופלא בדורו that Tehillim is permitted; he cites a Medrash that Yackov Avinu completed the entire Sefer Tehillim at night.)
4) The Tzitz Eliezer (ibid) writes that even if Tehillim were included, this would only be דרך לימוד - learning the Pesukim. It is permitted דרך תפילה ובקשה - in a supplicatory fashion.
5) Even the stringent possum agree it is permitted on Shabbos and Yom tov. (שו"ת יביע אומר חלק ו' חלק אורח חיים סימן ל אות ב)
6) According to all Poskim, it is permissible after חצות. - [Kaf Hachaim (237:9)]
Malbim [Artzos Hachaim] proves from Yuma 18b, Shabbos 12a that it is מותר מעיקר הדין.
Sdei Chemed (Or Li §40), for discussion of what is included in the issur.
[Summary: Some Poskim forbid reciting Tehillim between nightfall and Chatzos. However, many others posit that reciting Tehillim does not constitute the prohibition of learning Mikra at night.]
(When I asked a leading Posek, he paskened that one should only do so in case of Choileh.)