15 month old slept through the night for first time ever last night, but now looks more tired than ever! Why?

[mumatuks]

DS2 who is 15months slept through the night last night. He went down in his cot at 9pm and woke at 7am this morning.
He's happy in himself and "chatting" to his big brother, but his eyes look heavy and he keeps yawning!
As I said, this is the first time ever he's slept through, as he always woke up for a comforting mummy suckle, or a cup of milk. I'm really happy he's sleeping through, and hope this is a sign of things to come, what with baby3 on the way.
I'm just curious as to why DS2 looks so tired after sleeping for 10 hours?!
TIA. x

He will be fine shortly, just takes longer tp wake fully after a good nights sleep.

Thanks WF. I think I'm still rattled that he wasn't there next to me this morning!
I must say, I appreciated a full 8 hrs sleep!