For healthy normal people, when you travel your circadian rhythms reset your body's internal clock and sleeping schedule based mostly on sunlight exposure. It takes between a few days and a week to adjust (this is exactly jet lag).

There is absolutely variation between time zones extremes. The Spanish, for instance, notoriously eat and stay up "late" compared to the Polish or Germans. All three countries are in the same time zone, despite the sun setting about 2 hours later in Spain.

Social habits do matter (the Spanish schedule is somewhat more delayed than the 2 hours you might expect from sunlight alone).

If you'd like to look up more on this topic, you can read about light therapy, sleep hygiene, shift worker syndrome (where normal people try to sleep later than natural), and delayed sleep phase syndrome (where night owls try to sleep earlier than natural).

The assumption here is probably based on the sleeping pattern of people.