What causes severe fatigue that makes you sleep almost all day?

Like you are so tired that you fall asleep at 9 pm wake up at 12 am go back to bed and wake up at 7 am then go back to sleep at wake up at 11 AM and still feel tired throughout the day. What disease is this caused by? Is this dangerous enough to go to the emergency room? Does it mean you are dying?

What causes severe fatigue that makes you sleep almost all day?
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