The oldest dynamic force shaping life on Earth

Water and air are often described as the foundations of life, however, it took 2 billion years for the Earth’s atmosphere to form. The sun’s cycles of light and dark, warming and cooling have been a life shaping force since Earth’s beginning. Adapting and optimizing survival within sunlight’s dynamic cycles is at the core of all earthly life processes.

Given the fundamental role cyclical sunlight has played in forming life’s adaptive strategies, it follows that sunlight and temperature cycles would play a significant physiologic role in human adaptation and survival. And they do. Growing scientific evidence from a wide range of fields such as molecular biology, comparative and cardiovascular physiology, and neurophysiology is revealing that light and temperature cycles regulate the timing and coordination of nearly every physiological and behavioral function of the body.

As a group, the physiologic processes linked to and coordinated by sunlight cycles are generally termed circadian rhythms. Circadian comes from circa, meaning about or around and dia, meaning day - the physiologic processes that occur rhythmically or cyclically on a daily, approximately 24 hour basis.

Sunlight and temperature cycles regulate the timing and effectiveness of basic behaviors such as sleep, physical stamina, cognitive performance, alertness and appetite - organizing each of these behaviors into a circadian rhythm. These same cycles also regulate and coordinate the circadian rhythm of less apparent functions such as metabolic rate, blood pressure, heart rate variability, cell division, cellular longevity, immune function and hormonal balance. The evidence for the critical role of physiological circadian synchronization based on sunlight and temperature cycles is growing rapidly. This same evidence is also revealing relationships between loss of physiological synchronization and many of the chronic diseases afflicting contemporary life.

Dynamic Daily and Seasonal Cycles of Sunlight
The Survival Value of Predictive Adaptation
Circadian Synchrony and Optimal Physiological Function
New Approaches to Wellness and Treatment
Integrating Circadian Synchrony with Contemporary Life
Afternoon Tips for Circadian Synchrony
Integrating Circadian Synchrony at Night
American Society for Photobiology
CIrcadian Rhythm
Healthfinder Sleep Resources
International Dark Sky Association
Kavli Institute conference: Biological Switches and Clocks
National Sleep Foundation
Sleep Research Society