How much sleep does the manual say adults need to maintain alertness?
Eight to nine hours is what the manual gives, against the less than five hours a day the long-haul study actually measured.
This answer is not the same in every state
New York's manual says the average person needs seven or eight hours of sleep every 24 hours; the AAMVA model edition, California and Oregon say adults need 8 to 9 hours. Serve the state-scoped version where one exists.
Go by the manual your state publishes — that is what your examiner grades.