Why do I wake up tired even after eight hours in bed?

Deep sleep is when your body and brain actually repair. Some people's wiring shortens those windows, so they clock the recommended hours and still wake unrefreshed. The number on the clock looks fine; the recovery underneath it is short.

You can protect deep sleep with the basics - cool dark room, consistent timing, no late caffeine or alcohol - but if you have always woken tired despite enough hours, your architecture, not your effort, may be the variable.