Génesis 28:17 Halajá: Kitzur Shulchan Arukh

You should express grief in the manner of your sleeping. For example, if you are accustomed to sleep on two pillows, you should sleep only on one. Some people have the custom to sleep on the floor during the night of Tishah beAv, and to put a stone underneath their head as a reminder of what was said, "And he took some of the stones of the place etc.," (Genesis 28:11). Our Sages comment that Jacob foresaw the destruction of the Beis Hamikdash and said, "How frightful" etc. (verse 17). It all depends on the person's attitude.²A pregnant woman or any weak (or sick) person is not obligated to do any of these things. (Ibid. 555:7)