Interestingly, the new NH 777 staggered configuration in business class actually has a "Do Not Disturb" light that one can turn on, and not to be bothered by the FA!
https://www.ana.co.jp/wws/us/e/asw_c...ess_staggered/
Scroll to the bottom right.
In the old days, I recall having "Do Not Disturb" or "Don't wake me up for meal" type decals that one can put on the headrest area to warn FA not to bother you.