It depends on how long you will be in Japan and for what purpose. For my visits, I have found it actually makes more sense to stay awake through most of the flight, just cat-napping. I then arrive somewhat tired and go to bed at a normal time for Japan, getting up the next morning as usual. If you want to use Ambien, do it that first night in Japan. Doing that, you cut down on the time that you feel like falling asleep in the middle of the day. My first visit years ago, I tried it the other way, finding myself almost uncontrollably napping in the middle of the afternoon and not sleeping well at night. Doing that, I take days to get over it. Works better for me to get it over with right away on the plane and that first, miserable night.