Why Is Sunlight an Important Factor to the Sleep-Wake Cycle

Why is sunlight an important factor to the sleep-wake cycle?

A) Our bodies have become accustomed to waking up when the sun rises.
B) The lack of sunlight causes our bodies to stop the production of melatonin,which allows us to become sleepy.
C) Our body sends signals to the brain that reduce our sleepiness when it registers sunlight.
D) Our circadian rhythm is preprogrammed to operate on a 24-hour clock so it doesn't matter when the sun is out;our body will sleep when it's used to sleeping.