I would guess anywhere that includes India and China in the measurement.
Take whatever time they go to bed in their cultures and pan out to the larger population masses either side ?
Given it's about 9hrs between England (far west) to Japan (far east) then somewhere around when it's +6 GMT it 2 in the morning (assuming most the population sleep between 8 till 8 (don't forget children
and it makes it easy)
I think that creates a large sleeping population as goign east from china to the Americas is to far and the population I doub't is as high as that covered going west.
Of course not made do easy by peoples time zones not matching up with time xone positions
(just searched for a map and noticed the 'mess' )http://www.timeanddate.com/time/map/
Well compared to other huge countries, China is a little simpler, as they only use one time zone. They're currently 8 hours ahead.
India, I think, has some variance in time zones, but I seem to think they only vary by an hour or so. Think they're about 5 hours ahead.
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