I like the idea of having side effects for having your character depressed but not to the point of randomly injuring yourself as it may be hard for the player to understand what is happening especially if they are hypochondriacs. I do however like the idea that tasks would be performed slower or given up on depending on how deeply into depression the character is and that the learning process would be slowed and the being sleepy more often with sleep having a lower effect at relieving the tired moodlet. I also think in addition to these things that being depressed would also increase your risk of infection and getting sick by other means as a lot of stress tends to lower your immune system.
