For many depression is actually the symptom of chronic exhaustion , a runaway positive feedback loop of sleep deprivation and neuro-physiological brain fatigue.
What do I mean?
When you are sleep, there are two very important stages of sleep. One is REM sleep, which is what we know as dreaming. The exact function of dreaming is a kind of de-excitation / de-activation mechanism for neural pathways that become activated during the waking day. Example, your boss is stressing you out at work, your brain has many circuits activate with thoughts of confronting him, punching him, trying to appease him, and so on - your brain processing how to handle the situation. You then REM dream about it because your brain is de-activating all of those pathways, purging itself if you will.
The other main stage of sleep is a restorative stage, where the brain carries out maintenance and cellular repair and consolidation of memory etc. It is this type of sleep that results in the well rested feeling you might get in the morning, after a really good nap and so on. You feel good because your brain has returned itself to an optimum physiological state, having become degraded from the days processing activity.
Its a bit like restarting an older windows XP era computer, if you didn't restart every so often they'd eventually slow down so much they barely ran at all.
Im oversimplifying the stages of sleep but that is the essence of it. Now, depressed people get stuck in a particular type of feedback loop. They become stressed for whatever reasons (work, relationship, inceldom) which causes lots of excitation of neural pathways in their brain, the brain has to then de-excite these during sleep. However if you only sleep 8 hours, the brain spends more time in the REM stage than it ought to - at the cost of restorative phase sleep. Its why if you wake up after a night of vivid dreaming, you often feel really tired and unrefreshed. Your brain hasn't repaired itself physiologically from the previous days processing.
This reduced function means you aren't able to cope with the stresses you face during the day, they become worse, you become more stressed. You REM even more, you deep phase restorative sleep even less. You wake up even more tired. Stress snowballs until you feel things are spiralling out of control. Even if you do get a full night of sleep, its not enough because so much time is given over to REM as a result of this out of control feedback loop.
You become chronically exhausted, because you aren't truly sleeping at night, you are functioning on the equivalent of only an hour or two of sleep a night. You got to the doctor, they ask you questions like "Do you feel down? Do you have difficulty sleeping?" and before you know it you are bundled to a psychiatrist and prescribed anti-depressants which fuck with your brain chemistry. People kill themselves on SSRI's.
