Photosensitive proteins and circadian rhythms originated in the earliest single-celled life, protecting replicating of DNA from high ultraviolet radiation during daytime. Nighttime is the right time: genic replication relegated to the dark.
Light-based rhythm regulation varies widely among species. Tadpoles and other amphibians detect light via pigmented skin cells, thus adapting camouflage to different backgrounds. Sparrows sense light through their feathers, skin, and bone.
It is the color of light, not its brightness per se, that the mammalian system uses to set its circadian rhythms. In the eyes of mammals, retinal ganglion cells (RGCs) detect higher wavelength (blue) light as part of the mechanism for maintaining biological rhythms. RGC cells may work even in blind animals.
Like many animals, the human body relies upon sunlight to maintain its biological clock. Without sunlight, there is a natural tendency for biological rhythms to adjust such that sensed time shortens: 3 hours may seem as 1. This temporal condensation lessens stress, as being without sunlight for extended periods when the body expects it is stressful.
Most bodily functions are regulated by bio-clockwork. Through genetically encoded protein production, the body ramps up metabolism in the morning: rise and shine.
Levels of bioclock proteins rise and fall during the day, slowing biological functions at night. Blood pressure drops, heart rate slows, mental processes wind down: time for bed.
The biological clock gets jittery with age, often causing the elderly to have trouble sleeping. Shift workers, who have jobs outside the biological norm, are at a much higher risk for certain diseases because their circadian rhythms are chronically out of whack.
Jet lag is a common phenomenon for long-distance air travelers. One of the best ways to set the bioclock to the current time zone is to eat at local mealtime. Insulin release helps put hormones on the right time track.
Daily cell cycles are fundamental to the biomechanics that control cell growth. Egg cells have a biological clock with different biomechanical responses than ordinary somatic cells.
Bad circadian rhythm has profound effects on health. Cancer and diabetes are related to metabolic cycles that are bio-rhythmically controlled.