DS, 5 months, is a pretty terrible sleeper, up every two hours, some times more often, although he does occasionally let me have a three hour stretch
He usually ends up in our bed between 6 and 7am, as he won't sleep in his own bed any later than that, but is still tired and grumpy. When he gets in with us he often sleeps til 9am.
Last night, he managed a 3 hour sleep, followed by an almost 5 hour sleep, which was great. The problem was, after I put him back down (at 5.30), he only managed to sleep for about another 20 minutes before waking up, giggling and cooing to himself in the cot. After listening to it for a while, I knew he wouldn't go back to sleep, and brought him into our bed again.
Is it possible that because he hadn't been up and down all night as usual, after his 8 or so hours, he'd actually had enough sleep for the night? He doesn't really nap in the day either (2 or 3 fifteen to twenty minute stretches, occasionally one nap will be a little bit longer, maybe up to 40 minutes), so this is really it til bedtime now. I do worry that he seems to be able to get by on much less sleep than I do!
Ds2 is like this - he is genuinely perfectly happy on very little sleep (10 hours at night, but awake for 3-4 feeds in that time, and no more than 60-90 mins in the day). I'm not sure it's terribly good for them (sleep being essential for developing bodies etc), but of they're totally awake and perfectly happy o'm not entirely sure what you're meant to do about it!
Ds2 is 6.5 months and has been like it since birth, btw.