Causes of Excessive Sleep in Young Adult

My 19-year-old son requires more sleep than normal. For the past four years or so, he is almost always tired. He only feels good if he gets 10 or more hours of sleep. This past spring he had mononucleosis, which would explain his tiredness at the time, but nothing explains why it lingers. He has had blood work done but doesn’t seem to get any answers. Any suggestions as to what the problem is or what steps we can take next?

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