Circadian rhythm—our endogenous biological clock—has synchronized life with Earth’s rotation for billions of years, from bacteria to humans. How does this specifically affect humans?
Light enters the eye through the retina, specialized brain tissue that converts light photons into electrical signals. These signals travel via the retinohypothalamic tract to the suprachiasmatic nucleus, abbreviated SCN. The SCN is our body’s master clock. This bilateral structure of approximately 20,000 neurons sits in the anterior hypothalamus, directly above the optic chiasm. This location allows the SCN to receive direct input from specialized retinal ganglion cells containing melanopsin, a photopigment that is distinct from rods and cones used for vision.
Let’s pause and clarify terminology. Hormones are chemical messengers produced by glands and tissues that travel through the bloodstream to cells and organs, regulating nearly every bodily function. Glands are organs that produce substances including hormones, digestive juices, sweat, tears, saliva and milk. Endocrine glands release hormones directly into the bloodstream, while exocrine glands release substances into a duct or opening to the inside or outside of the body.
Light signals reaching the SCN activates elegant molecular physiology. The SCN communicates with the anterior pituitary gland to produce adrenocorticotropic hormone (ACTH). ACTH triggers the adrenal glands to produce cortisol, a steroid hormone promoting wakefulness, raising body temperature, and mobilizing glucose. Anyone who has experienced sleep difficulties while taking steroids has experienced cortisol’s effects.
During darkness, the SCN signals the pineal gland through the sympathetic nervous system. The pineal gland is a small deep brain structure outside the blood-brain barrier, allowing direct hormonal access to circulation. The pineal gland produces melatonin, which promotes sleep and lowers core body temperature. Melatonin and cortisol form a reciprocal relationship defining our circadian rhythm. Beyond sleep regulation, melatonin regulates body temperature, metabolism, and hormone balance. It reduces inflammation, possesses antioxidant properties, and strengthens immune function by enhancing T-cell activity.
Here’s what’s remarkable: nearly every cell in our body contains this same molecular clock machinery. Peripheral oscillators exist in the liver, heart, kidneys, and isolated fibroblasts. The liver clock regulates glucose metabolism and drug metabolism enzymes, explaining why medication efficacy varies by administration time. The SCN synchronizes these peripheral clocks through hormonal signals, autonomic nervous system activity, and feeding-fasting cycles.
This complicated system isn’t merely about sleep—it’s evolutionary physiology that coordinates metabolism, immune function, cell division, and DNA repair across time. From the photosensitive retina to the deep brain SCN, through the sympathetic chain to the pineal gland, and ultimately to every cell, circadian rhythm orchestrates our physiology with temporal precision. Understanding this architecture reveals why circadian disruption affects us profoundly. In the next part, we’ll explore what happens when this elegant machinery malfunctions.
Music written and produced by Kenny Mihelich. Western Slope Skies is produced by the Colorado Mesa University Astronomy Club, the Western Slope Dark Sky Coalition, and KVNF Community Radio. This feature is written and shared by Dr. Kate Fedack.