Depression seems pretty clear to me
1. You need to be using your brain and body to their full capacity in pursuit of fufilling your biological imperatives - survival and reproduction
2. If you falter from this path and fall into a state of waking hell, you should be caught by a safety net of moral support structures of family, society, religion which can help you retructure your life. These structures are sadly being eroded.
3. Sometimes if all else fails, there could be antidepressants as a last resort. However all options - finding meaning and fixing sleep, diet, exercise, sex, etc - should be exhausted first