When crayfish are kept continuously in the dark, even for four to five months, their compound eyes continue to adjust on a daily schedule for daytime and nighttime vision.
Day and night were divided into the same number of parts, so that except at the equinoxes, days and night hours were unequal; and then of course the length of these hours varied with the seasons.
Men, these days, are embracing fatherhood with the round-the-clock involvement their partners have always dreamed of—handling night feedings, packing lunches and bandaging knees.
Humans are pegged to a 24-hour cycle. We're locked into it not just by day and night-there's the master timepiece in the brain called the circadian clock.
These periods became known as temporal hours, because their duration varied according to the changing length of days and nights with the passing of the seasons.
Also, sleep scientists know that even a small variation in people's circadian cycles gets amplified by such factors as when in their cycle light strikes the eye.