We all know that the normal hu...7 hours wakefulness.

Why is the question of "how easily people can get used to working at night" not a mere academic question A: Because few people like to reverse the cycle of sleep aud wakefulness. B: Because sleep normally coincides with the hours of darkness. C: Because people are required to work at night in some fields of industry. D: Because shift work in industry requires people to change their sleeping habits.