Personally, I usually sleep exactly 8 hours, and am naturally on a pretty regular schedule. But I used to sleep 4 hours, get up and do things for a few hours, and then sleep another 4 hours.
Most people should get 8-9 hours of sleep, but some people need more or less. Also, if the quality of the sleep isn't good, people need more time to get the full 8 hours. People with sleep apnia, for example, may wake up a hundred times during the night. In winter, we're designed to spend more time in bed, with a period of time in the middle of the night where we are 'between the worlds'.
A lot of people are really grumpy about being awakened. My NT partner is like that. I leave him the hell alone <grin>.
Ask him what he wants to do about things like dinner. Also, if he wants to get on a regular schedule, he needs to be a little sleep deprived to be able to sleep at the 'right' time, until he gets used to it.
There's also a normal biological thing that makes people in their teens and early twenties be 'night owls'. Getting up early sucks.
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"Yeah, I've always been myself, even when I was ill.
Only now I seem myself. And that's the important thing.
I have remembered how to seem."
-The Madness of King George