Describe the biological process underlying the circadian rhythm. Be sure to add

Bright light in our environment enters our eyes and reaches our retina. The photoreceptors within the retina let us know how much light is in the environment and send that information down the optic nerves to the suprachiasmatic nucleus. The suprachiasmatic nucleus is housed in the hypothalamus within our brain. The hypothalamus (from Chapter 2) controls the endocrine system. The pineal gland, which is part of the endocrine system, either secretes a hormone called melatonin or does not depending on how much light is in the environment. In low or no light, melatonin is secreted, which will cause a person to get sleepy. In brighter light, no melatonin will be secreted, which will help the person maintain wakefulness. Thus, we feel sleepy when it is dark and typically more awake in a well-lit environment.