Does she have her 2yr molars yet? My little girl has been in her big girl bed for about 4-5 months now, and just the other day started waking at like 3am. I could not figure out why she wouldn't go back to sleep. I've learned that if I can explore the possibilities with her until she knows I know why she's up, she goes back to sleep. Wierd. But, for a couple nights it was her first 2yr molar cutting through. Then it was that she actually recognized the need to go potty in the night for the first time. About this age (we're at 25 months, but a little advanced), they start recognizing different things more and start being more and more curious. So you might just try asking her (even at 5am) what do you need, do you need water, do you need to potty, do you have an owie, do your teeth hurt, did you get scared, do you just want a hug, are you COLD; things that would prompt her to tell you why she is awake.
Then don't keep her up, resolve the problem as quickly and minimally as possible and put her straight back to bed. If she wants water for instance, let her have a sip or two, then put her to bed. Trust me, she doesn't need the whole cup, just a sip to cure a dry mouth. Otherwise, you'll be up again in an hour for a potty break or diaper change!! If she just wanted you, hold her until she is really conked, then put her back to bed and go back to bed (usually takes about 5-10 minutes). She won't need this all the time, just needs to know you're there when she does.
Also, be aware of the cold thing. It's been proven that if we are cold in the night, we tend to have bad dreams (which they say kids start having dreams about 18-24 months). We often miss that having the AC on so high in the summer is ok because it's so hot, so we still dress summery. If you do, put socks on her at night to help. Otherwise, I use the lightweight one-piece pjs with the long sleeve arms and footies. They work great to keep her from getting too warm and also to keep her warm enough to sleep well.
Generally, resolving whatever the real need is conquers the issues. We're back to sleeping through again after just those two short-lived interruptions. :)
One last thing: my parents made the mistake of telling me that when it's dark we sleep, when it's light we wake up. They daylight savings time hit, and I never could sleep past sunrise, regardless what time it was. At the first hint of light I was up and ready to go. I went to my neighbors house and run their doorbell at 6am one summer to see if my friend could come out to play. My neighbor brought me home, woke up my mom, and told me never to come over again before 9am. :D Didn't matter what time I went to bed either. It lasted until I was like 19 or 20. Love those foundations. Anyway, if she's doing this in the summer when the sun rises so early, you might consider putting darker shades in her room (or something over her window) and see if she'll stay asleep longer. Then you'd at least know if she's picked up that concept somewhere.
Oh, and I sometimes have to stay in the room when I put her to sleep for a little bit still (occasionally), I'll rest in the rocking chair; like a little nap for me before going to clean up the house and get me ready for bed. But she has sporadic nights. I was just happy to get past the falling out of bed! Even with a rail, she figured out how to push it over so she could get out and then she would fall out. We gave up on the rail long ago, but she doesn't so much roll out anymore. :)
Best to you.