I'm probably telling you something that you know, but young child wake several times a night, but automatically put themselves back to sleep. Well for some children the more sleep they've had the less sleep imperative they have to go back to sleep, even though they need a little bit more, iyswim.
DD does this, it seems to be a combination of not finding it easy to go back to sleep even though she's still a bit tired, combined with low blood sugar, which equals some bloody awful behaviour far too early. If we get her up, she's very cranky indeed, tantrums over the slightest thing
So, we give her a 5 oz bottle of juice or milk and tell her it's still night-night time, it's still dark and mummy and daddy are sleeping. It puts her back to sleep for between 40 mins-1 hour 20 and when she wakes again she's much sunnier. It didn't happen immediately but it's pretty infallible now.
We've haven't co-slept since I stopped breast feeding at 6 months so coming in to mummy and daddy is very exciting and she doesn't go back to sleep, so this works best for us, though I think we're going to wean her off expecting the bottle soon.
Do have a bit of patience and hang around, more people will reply. Parenting is just sleepier than over sections. Give it 24 hours and you'll have a few more responses (probably some of them saying that giving juice/milk at 5am is irresponsible 
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