I've finally found what's wrong with my teenage son. Apparently it happens to about 7% of the population at some time and is more common in males. Because of previous depression his whole sleeping waking pattern has out of synch and he is awake for long periods in the night and asleep all day. What is exciting about this is we now know how to make it better.
Nightime routine
low light, cool room, hot bath, no caffeine drinks within 5 hours of sleeping, no computer or television within 15 minutes of sleeping, light supper.
No vigorous exercise before bedtime
If he can't sleep, he should only do things that are low light and calming such as reading or playing music.
In the morning very bright light for half an hour and on getting up, after breakfast vigorous exercise, and high protein diet.
It seems to be working though we are hoping to get therapy too to help him relax.

You must be relieved to have a diagnosis and a way forward. I hope it works quickly.

Hope he gets better soon. I had this once, and the diet/regime didn't work for me and I ended up being put on some pills to regulate my sleep cycle.. Can't remember their name though.

Good luck!! :D

They tried pills. It had no impact whatever. He's not exactly right on time in the morning but being up at all is a great improvement and gives us something to build on. Getting things he has to be up for at the weekend is helping too.

My husband used to be bad for that when we first got together (he was nearly 17)