It was not until as recently as the last half century that the complexities of biological timing processes within organisms began to be unravelled. The common denominator between both living and non-living clocks is the role played by light. For 4 billion years, the presence of the sun and the continuous rotation of our planet around its own axis has resulted in the ever changing cycles of light and dark. Consequently, cellular clock mechanisms have evolved light sensitivity to provide organisms with the ability to anticipate periods of activity and in doing so, optimize survival. While some cultures and religions still revere the power of the sun, there is no denying the power of light on our physiology and behaviour. Short wavelength blue light is captured by the mammalian eye and directed via nerve fibres to a master clock in the brain, that in turn signals timing information throughout the body. In this way, almost every cell in the body functions with a rhythm reflecting the 24-hour light-dark-cycle. The ability to keep time with the environment provides an integral homeostatic role for the circadian system, ensuring all cells and tissues work synchronously and in harmony with the rhythms of the external world.

However, things were simpler for our Stone Age ancestors – the rising and setting of the sun set limits on what could practically be accomplished within daylight hours. The darkness of night lit only by the yellow glow of firelight permitted the regular rise of the rest-inducing hormone melatonin. In contrast, the sun’s blue wavelengths by day create an alerting effect by suppressing melatonin production and preparing the body for activity. In today’s 24-hour society, thanks to electric light, shift work, and jet travel, inappropriate light exposure plays havoc with our internal timekeeping system. Dampened and disrupted internal rhythms are key contributors to most serious human diseases and behavioural disorders. A failure to respect the power of light is having a negative impact on our health and that of our animals.

On this winter solstice, a time that marks the return of longer daylengths in what has been a very dark year, it is worth reflecting on the spiritual significance this may have had for our ancestors. In addition, a new practical appreciation of the mechanics of our biological clocks and the role light plays in our health and well-being should have us looking to the solstice dawn with renewed hope and positivity.

Barbara A. Murphy, PHD