Why you wake up tired after 8 hours of sleep usually comes down to this: the hours you spent in bed weren't fully restful. Sleep can be broken up by things you never notice (noise, light, heat, alcohol, stress), disrupted by a sleep disorder such as sleep apnea, or out of step with your body clock. Sometimes the cause isn't sleep at all but a health condition such as anemia, a thyroid problem or depression. Some morning grogginess is also normal. It's called sleep inertia, and it typically fades soon after you wake up. This guide walks through the most common causes, how to tell them apart, what you can change tonight, and when it's worth talking to a doctor. Quick answer: the most common reasons Fragmented sleep: you wake briefly during the night without remembering. A sleep disorder: sleep apnea, restless legs syndrome or insomnia. An irregular schedule: your sleep window doesn't match your body clock. Alcohol, late caffeine or heavy late meals that dist...