TL/DR: If you even think there is an outside chance that you might possibly, maybe have sleep apnea, get tested.
About 8 months ago I started putting on weight, dealing with depression, diabetes getting out of control, generally not caring about anything. Could not pull myself out of it, started seeing various doctors, changed diabetes meds, tried some anti-depressants. Nothing was working. My diet didn't suddenly go downhill, but I was always tired and still putting on weight. I even changed jobs thinking it was all stress related. My new job is the least stress that one can have and still call it a job. Things still got worse. Now I was having problems with memory and concentration. I would get lost driving home, could not pay attention in meetings, would lose track of hours at a time. Depression so bad that I would just sit and cry. More doctors, more drug changes.
A friend suggested that I might have sleep apnea, so I asked my doctor if that could cause any of my problems. He says it could cause ALL of my problems. I had a sleep study and they found that I had an AHI of 37, my blood oxygen levels would drop to the low 70's. Severe Sleep Apnea.
I started using a CPAP a month ago. Holy Fuck, life is great! Within 3 days I had so much energy that I was doing two workouts a day just to burn it off. My thinking cleared up. Confidence in myself that I haven't felt in 15 years. I'm down 10 pounds, yay I don't have to go buy fat clothes! I'm dating again and motorcycles are fun again.
I have basically been walking around dead for at least 10 years. It seems sleep deprivation and oxygen starvation really are bad things. Combine those two with glucose levels that make your brain function like a drunk person, and it gets worse. Go get a sleep study people! I might even get to the point where I can form proper sentence and paragraph formation that does not drive the grammar nazi's crazy.