It's possible he has some kind of sleep disorder which is cause him to not feel rested, no matter how much he sleeps. Sleep apnea or restless legs, for example.
I've also been told by my sleep doc (I have just about every sleep disorder there is
) that sleeping *too much* can actually make you more tired.
Also, are you sure he is actually sleeping all that time? I had terrible insomnia as a kid and would lie awake for hours.
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Sharing the spectrum with my awesome daughter.