https://astralcodexten.substack.com/p/sleep-is-the-mate-of-death

Melancholic depressive patients report that they feel worst in the morning, just after waking up, get better as the day goes on, and feel least affected in the evening just before bed. Continue the trend, and you might wonder how depressed people would feel after spending 24 or 36 or 48 hours awake. Some scientists made them stay awake to check, and the answer is: they feel great! About 70% of cases of treatment-resistant depression go away completely if the patient stays awake long enough. This would be a great depression cure, except that the depression comes back as soon as they go to sleep. There's a lot of great work going on to figure out how to make cure-by-sleep-deprivation last longer - see the Chronotherapeutics Manual for more details.

But forget the practical side of this for now. It looks like sleep is somehow renewing these people's depressions. As if depression is caused by some injury during sleep, heals part of the way during an average day (or all the way during an extra-long day of sleep deprivation) and then the same injury gets re-inflicted during sleep the next night.