What is the primary stimulus that regulates the "biological clock" that controls circadian rhythms?

Question

Quizplus · Accepted Answer

The primary stimulus that regulates the biological clock that controls circadian rhythms is light. Specifically, it is the presence or absence of light that signals to the brain's suprachiasmatic nucleus (SCN) whether it is daytime or nighttime and helps to regulate on a 24-hour cycle when biological processes such as sleeping, eating, and hormone production occur.

What Is the Primary Stimulus That Regulates the "Biological Clock