While college students may sometimes be an exception, most humans sleep at night and are awake during the day. What maintains this relationship?
A) The eye relays messages to the brain about light levels, which synchronizes the circadian rhythms to the day-night cycle.
B) People get more exercise during daylight, which promotes alertness and wakefulness.
C) Ambient air temperature is cooler at night, which triggers the body to sleep.
D) Our internal biological clock is synchronized with the day-night cycle when we are born.
Answer: A
Rationale: One key brain structure in the regulation of the sleep-wake cycle is the suprachiasmatic nucleus (SCN) of the hypothalamus. Cells in the retina of the eye relay messages about light levels in the environment to the SCN. The SCN in turn communicates signals about light levels with the pineal gland. The pineal gland releases a hormone called melatonin, which peaks at nighttime and is reduced during wakefulness. Thanks to this system, light is the primary stimulus regulating our circadian rhythm and it tells us to sleep at night and stay awake for the day.
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