John Fischer once suggested to me that many of the best philosophers are mildly depressed: This gives them the lack of confidence necessary to recheck and rethink their arguments with paranoid care; it prompts them to toss what's less than excellent in the trash; it gives them a realistic appreciation of how someone opposed to their point of view might react to their writing. The average person generally accepts her first thought with blithe confidence and is satisfied to stop there until someone points out a flaw. The mildly depressed philosopher worries that her first thought is off-target or too simple, that there are important objections she hasn't considered, and that her opponent may be right after all. Consequently her thinking deepens.
There's much right in this, I suspect. Surely you can think of philosophers (I won't name any names!) who are rather too satisfied with their own work and their first thoughts in response to challenges, whose philosophy would profit from a loss of self-esteem! (Perhaps in their early careers, before they earned their flattering students and editors, they were rather more depressed?) And contrary to what one might superficially think, a depressive lack of confidence is quite compatible with the seemingly arrogant conviction, essential to the boldest, most creative philosophy, that every other scholar in the world (and Kant) is farther wrong than you.
On the other hand, there's something to be said for euphoric philosophy, too -- philosophy that strikes out in new directions, without too much looking back, philosophy that doesn't detain itself overmuch with fine distinctions and robotic consistency. And of course even mild depression is enervating, making it hard to take up the bold project or even just to sit down and write or revise what one has already planned out. The inner critic speaks too loudly after each sentence, each paragraph -- they don't come out, or half come out, or come out and get deleted.
The best pathology for a philosopher is probably mild manic-depression. The ephoria, self-confidence, and energy of a mild hypomania can drive the drafts, enliven one's thinking, encourage new starts, new directions, bold ideas. The subsequent depression puts one's feet back on the ground when it comes time to revise, rethink, and often just completely abandon the thing. The philosopher's ideal condition is one of gentle fluctuation.
The implications of this for the proper use of caffeine and diurnal rhythms I leave as an exercise for the reader.