5yr old restless sleeper and wakes before 5am

A question for parents....
My son is 5yrs and in prep, the problem is his sleep! He gets up often before 5am and so quiet we dont realise until 6am sometimes, he always has the blackest rings under his eyes. He is so restless all night, sleep talking and tossing and turning. During the day he is a lovely boy and loves school, eats well and plenty of fruit and veg. But by 5pm is totally cranky. We don't know what to do! He has a clock and knows not to get up before 6am but says it takes too long to go to 6am. His room has a blind and curtain so its nice and dark. He has a fan going so it blocks out noises. Help please!