During my stay in London, I had a habit of writing down the psychological time (the time I felt it must be) when I suddenly woke from sleep. Then I would also check the exact time. One day, I discovered that the time written on the wall made up a circulation of a day. I took a photo to remember "this day"—and the "one day" was actually recorded over multiple days.
*The documentation of my sleep experience also includes a room of darkness.