What the empty hours get filled with
The fourth box in the loop is the one people underestimate. Time alone with your own thinking is not neutral time when you are depressed.
What the research says
Nolen-Hoeksema S, Wisco BE, Lyubomirsky S (2008). Rethinking rumination. Perspectives on Psychological Science, 3, 400 to 424 doi
Going over a problem repeatedly prolongs and deepens a low episode rather than resolving it. What separates it from actual problem solving is whether it ever ends in an action. If it never ends in an action, it is not working on the problem, however much it feels like it is.