Diurnal Variation in Rumination
Rumination, the circling and re-circling of the same thought, is not spread evenly through the day. It runs higher in the morning, dips to its lowest point in the middle of the afternoon, and climbs again towards the evening. Keisuke Takano and Yoshihiko Tanno established that shape in 2011, and the part that matters most is that the evening climb is steeper in people scoring higher for depression.
What it is
The study behind it works by catching people in the act rather than asking them afterwards. Participants carried a device that pinged them eight times a day at semi-random intervals for a week, and each time they recorded what they were thinking about and how they felt. A composite of how self-focused, how unpleasant and how uncontrollable the thinking was traced a curve across the day that sags in the afternoon and rises towards the end of it.
That is the best single piece of evidence behind the common sense that the thinking gets worse at night. It is also a good deal narrower than the sense it supports, and the gap between the two is where the folklore lives.