I would imagine some guests would be getting up for morning flights between 5-8 am and it may be a bit noisy around the cabins if you're trying to go to sleep. The cabins aren't fully enclosed with solid doors. Reviews also note that it is consistently fully booked and I believe cleaning is done mid-day so walk up availability isn't likely.

This thread might be of interest:
Arriving Haneda at 5am. What should I do?