I'm very curious about this, because I know that if I don't have something regularly scheduled for morning or early afternoon I tend to sink into depression, and now I'm wondering if I could perhaps be getting too much sleep. I had thought it was either a) getting sleep at the wrong times (I'll tend to stay awake later each night) and thus missing out on sunlight, or b) that regularity itself prevented my depression. This would also explain it, though.
The best way I can describe the effect is that for me it's like hitting a "reset" button in my brain. At first there's the tiredness and irritability that always go with not sleeping, but eventually I get a "second wind" and I feel normal, calm, and content. This generally lasts the whole rest of the day, until I fall asleep again.
The sleep deprivation treatment for depression used by some doctors in the '70s involved either total deprivation or waking the patients up around midnight, so that they slept only a few hours. I think I read that both worked equally well, but I have only tried the former. And as far as I know, nobody has figured out why it works for so many people.