Mishneh Torah Tefilah uVirkat Kohanim 7:2 mentions verses of mercy being read before going to bed. Shevuot 15b relates that R' Yehoshua ben Levi used to say (at least the first half of) Psalm 91, and the Tur (Orach Chayim 239) lists it among the texts customarily recited after the Hamapil blessing. Most prayer books add one more verse before (Psalm 90:17), and repeat the last verse, which is commonly known as Vihi Noam. However, we don't say Vihi Noam after the evening prayer on Tishah beAv when it falls on Saturday night (Shulchan Arukh Orach Chayim 559:2). Shall we then omit it at the bedtime Shema as well?