Well, I've got a kid who sleeps when she's sick. She doesn't whine, cough, wheeze or puke with most things, she just sleeps. She'll sleep for 36 solid hours when she's ill, waking only for water and the toilet.

Her sleep is usually less restful when she starts such an illness.
It's probably got more to do with the rhythm of sleep than anything, though. If he went to bed at 8 every night, his body would be used to planning around it... but because he isn't, his sleep cycle doesn't line up right. It's complicated, and of course without a lot more information I'd have no way of knowing which was the cause and which the effect (i.e. whether the rhythm is what makes him go to sleep later, or he goes to sleep later which made the rhythm different).
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