Changes in sleep patterns in kids are really common. Our 3.5 year old is a horrible sleeper and wakes-up every night (we have yet to figure out the reason why).
I haven't gotten a good night's sleep in about 10 years. I'm lucky if I get 3 hours of sleep before I'm either awoken by a kid or wake-up on my own (this was a problem well before I had kids). I've done sleep studies, and there appear to be no reasons other than I'm a light sleeper.
But, and please don't take this the wrong way, part of being a parent is having to deal with issues such as these. They're disruptive but temporary.
One thing my Mother-in-Law said she did with my husband was to put a sleeping bag in their room next to their bed. That way, if he got up in the middle of the night, he could come in there and sleep without disrupting everyone else. I guess he outgrew it in a few months, and it helped everyone get a better night's sleep.