The Relationship Between Light, Our Body Clock and Circadian Rhythms

Light Can Affect Our Body Clock, but Only at Enough Brightness

Circadian rhythms cycle and recur daily. The timing of the human body clock's day, however, will vary to the amount and brightness of light within our environment. As infants, we begin life in the body tied to the rhythms of our mother. As we emerge from the womb, our bodies begin their entrainment to the sun's path. This entrainment process takes a little time. Infants progress from almost all day sleep (with some feeding and crying in between) to periodic sleep. Gradually, the frequency of sleep decreases. Not randomly, mind you. Rather, a baby's sleep cycles decrease in daily frequency at a consistent pattern, influenced by geographical location (with respect to the equator) and the amount of artificial light present in the room. In other words, with each passing sunrise and sunset, our bodies slowly and methodologically entrain their rhythms to nature's cycles. By the time our bodies reach adulthood, our clocks have become quite rigid, exhaustively trained by so many daily cycles of the sun over the years. 

Over the past forty years, sleep researchers have been intensely studying our entrained circadian clocks by incubating and isolating people and animals. This has been accomplished variously, using caves and isolated chambers with and without the benefit of light. A host of mechanisms related to the body's circadian clockworks has been proposed. Various studies done with and without interaction with light have shown that melatonin rises with the dimming of lights. Melatonin levels also tend to fall with the body's interaction with even a little light. Body temperature also appears to be a major component of the melatonin surge. As the body cools with the dimming of lights in the evening, melatonin begins to pulse into the bloodstream. In the early evening, cortisol levels peak and then begin to fall off with the core temperature, guiding the body into slumber. As cortisol begins its rise around 3am, body core temperature begins to increase, as our body gradually prepares itself for a new day of activity. 

Our circadian rhythms revolve around the sun's path, but are influenced by many other factors. The sun's path orchestrates the body's rhythms through the activity of the pineal gland, the SCN cells and various clockwork genes, which together orchestrate the stimulation of hormones. Receiving light through the eyes is critical to the pineal's response. Yet light seems to stimulate the pineal with the eyes closed or blinded. Most assuredly there are various other receptors around the body that respond to the sun's radiation as well. Therefore, we can surmise that it is the electromagnetic activity of the sun; not merely the visible light spectrum that lies at the heart of the sun's influence upon the body. The sun's electromagnetic waveform mechanisms also adjust the body's rhythms on a daily basis. These adjustments are also expressed through physiological messenger pathways, which stimulate the secretion and activity of the various hor-mones and neurotransmitters, and their receptors. 

Cave studies like Kleitman (1963); Siffre (1972), and Miles et al. (1977) have indicated that the human body's daily revolution without the resetting mechanism of daylight is about 24.9 hours. This exact period has been debated, as researchers have also seen body rhythms cycle variously. In a study done by Folkard in 1996, a woman was isolated for twenty-five days without daily light cues. While her temperature cycle was close to twenty-four hours long, her sleep cycle was closer to thirty hours. This study indicated that over time, the clock tends to stretch out without any sun. It also indicates individuality among responses to a lack of daily sun radiation. Without the daily resetting mechanism of the sun, we might be going to bed later and later each night, and after a few days, we might be doing all-nighters and sleeping during the daytime. 

The consistent issue among these various studies on the body's circadian rhythms is how predictably the body's clockworks mechanisms reset with sunlight. Evidence Dr. Czeisler performed studies (1989) with the Naval Health Research Center on Trident nuclear submarine crewmembers. Sub operation schedules required crew to attempt to maintain an 18-hour body clock. Dr. Czeisler's results found this just was not possible. The onboard lights were simply too weak to entrain their body clocks to that schedule. Dr. Czeisler and others had previously established that bright light from between 7,000 and 13,000 (typical daylight) lux was necessary to produce such a dramatic resetting of the body clock (Boivin et al. 1996), but less than that didn't have the brightness to affect our cascadian body clock.