I tried a couple times to check in at ~6am, but were never able to check in earlier than 9am. (both times at Asian ICH due to early flight arrival). The thing is, often, if they let you check in early, they're essentially giving you a free night. I'm sure sometimes they can bend the rule, but be sure they are bending the rule... Also be careful when you try this that they aren't saying "Yes sir! I'll give you your room!", then you later get a bill for 1 extra night...

I have a funny story, that is quite the opposite - When is the LATEST you can check-in... I had a reservation at NRT Crowne Plaza at the end of a business trip. I believe the check-out time was 11am. The night before, I decided to stay up and visit the Tsukiji fish market, and then ended up getting to the Crowne Plaza a little after 10am. Due to the language difference, they were quite confused that I still wanted to check-in for 1 hour and shower. (Also, I wanted my points!) After manager intervention, they gave me a room key (a superior room no less!) and smiled. I showered and was out at 11:05am and went on to my flight.