Living organisms on this planet have adapted to the daily rotation of the earth on its axis. By means of "endogenous circadian clocks" that can be synchronized to the daily and seasonal changes in external time cues, most notably light and temperature. Most people anticipate environmental transitions, perform activities at biologically advantageous times during the day, and undergo characteristic seasonal responses. The effects of transmeridian flight and shift work are stark reminders that although modern technologies can create "cities that never sleep" we cannot escape the recalcitrance of endogenous clocks that regulate much of our physiology and behavior. Recent progress in understanding the molecular mechanisms underlying circadian rhythms has been remarkable. In its most basic form, circadian clocks are comprised of a set of proteins that generate a self-sustaining feedback loop with a free-running period of about 24 hours. One or more of the clock components is acutely sensitive to light, thus it can be synchronized to local time.
Levels of light and the Circadian Rhythm
The sleep-wake cycle, or circadian rhythm, in people is largely governed by exposure to light. Bright light enters the eye and follows the optic nerve to where the nerves for both eyes cross, and then stimulates a small bit of tissue that begins a cascade of chemical and nervous events stimulating wakefulness. In particular, a substance called melatonin is prevented from being produced by the stimulation of light. When people are exposed to low light levels or darkness, as in a Com Center or radar ARTCC room, the brain interprets this as sleep time and begins to produce melatonin and other substances that tell the body to begin to shut down. This change in the body clock has both physiological and psychological effects. The digestive system slows secretion of digestive...