Sleep

My lo has been sleeping for a little bit longer for his first chunk of the night, about 4-5 hours which is great but now he isn't going down until later and later. We put him down and he almost cat naps for 20-30 minutes and then wakes back up. We haven't done anything different to his routine so I'm not sure why. Any ideas on way we can get him down before 11/12 each night? I don't mind him waking up to eat etc I'd just like to get ahead of this and not form a habit of him going down to bed really late each night.