it's always time for coffee
You need a certain amount of sleep. If you sleep very little or a whole lot, you will feel very tired. I have no idea why. But I always sleep a ton, and I'm always feeling exhausted.
You absolutely need to have a physician answer this question, probably with a sleep study (You go to bed in a special room with a bunch of wires hooked to you) When I did it they discovered I stopped breathing about 15 times that night. It wasn't life threatening but sure as hell prevented a sound sleep. They got me what is called a C-Pap machine. It helps you breath by giving you a gentle air pressure in your nose. Now that I have one I sleep great and feel refreshed when I wake up on the morning.