Q. I have a problem Rov. I can’t usually fall asleep unless I thing of Divrei Torah I like. Is just thinking Divrei Torah prohibited during Tisha Beav?

A. Shulchan Aruch (O.H. 554: 3) quotes that it is prohibited even thinking of words of Torah during Tisha Beav.

Piskei Teshuvos (ibid.) permits just thinking of Torah when the learning is simple and superficial, and that applies even to topics that are permitted on that day.
He also permits writing superficial, simple small notes in order to remember the ideas for after Tisha Beav.

Horav Dovid Pam and Horav Aharon Miller Shlit’a opinions are similar.

Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a