When is USD/Yen most active?

Is it during U.S. market hours or during Japan market hours?

 

I dont know about the day but at night the are activity at Japan open (I think 20:00 New-York time) between 23:00-24:00 (after the asian break).

 

I think 90% of the time during the london trading session and into the US session. However, you may get activity on the JPY crosspairs during the asian session for obvious reasons