Hi, it sure does make it hard when the little darlings decide they want to be awake when we want to be asleep!!!
Ella (now 6.5months) started doing that about a couple of months ago to, usually just for no apparent reason. I would go into her, pick her up give her a cuddle and she'd go back off to sleep (expect for everynow and then she'd convince me she was hungry). Eventually i got sick of getting up 2-3 times a night so I let her cry a couple of times, usually not serious crying and it'd only last 3-5minutes, luckily because i hate letting her cry, then she'd give up and go back to sleep. Now TOUCHWOOD she sleeps 7pm to 6-6.30am. I think it seems to be about this age that they start going through some pretty big changes and really throw your/their routines about! I've certianly noticed that with Ella.
Good luck!!!! Hope you get some sleep soon! Let us know how you go.
Belinda, WA, lovely baby girl