Resilience and vitality through environmental synchrony

When we consider the relationships between sunlight, survival and resilience, we usually think about how light makes vision possible and how our ability to perceive the environment is critical to our survival.  While vision assists us in knowing what is in our immediate surroundings, many other sunlight sensing mechanisms have evolved to help us predict changes in the environment, especially changes that used to impact food availability and long-term survival like seasonal changes in duration of day and night.

A closer look at non-visual sunlight detecting mechanisms (ie photoperiodism) demonstrates that human physiology has evolved to optimize resilience and vitality through processes that are sensitive to the timing and the spectral distribution of sunlight.

One of the best understood examples of non-visual sunlight sensing is demonstrated by retinal cells that relay light information to the part of the brain, called  the suprachiasmic nucleus (SCN), that controls melatonin secretion and the the sleep-wake cycle. Light, especially from the blue part of the spectrum, increases brain activity associated with alertness. Darkness increases melatonin secretion which is associated with sleep.  I

In humans, light exposure at night suppresses melatonin secretion which delays or disrupts sleep depending on the timing and color spectra of the the light. From an evolutionary and adaptive perspective, using sunlight to regulate the wake-sleep cycle synchronized alertness, activity and sleep with the long days and short nights of summer and maximized the opportunities to gather food when it was most plentiful.

In addition to the survival advantages of increased food gathering opportunities during summer, the shorter sleep durations associated with summer trigger a hormonal increase in leptin and a decrease in cortisol levels that together increase appetite, food seeking and caloric hoarding.  Similar to bears’ late summer, massive consumption of calories in anticipation of winter, human physiology evolved a complex system for anticipating and adapting to cyclic food abundance and scarcity through non-visual sensing of sunlight and the relative duration of night and day.

In contrast to the effect of summertime shorter sleep duration on increased leptin and appetite, longer sleep durations that occur in naturalistic lifestyles during the long nights of winter are associated with increases in leptin, which in turn causes decreased appetite and decreased food seeking.  

The relationships between day length, appetite and food seeking are but one example of how physiological synchrony with the environment has evolved to increase survival and resilience.

Cajochen C, Munch M, Kobialka S, Krauchi K, et al. High sensitivity of human melatonin, alertness, thermoregulation and heart rate to short wavelength light. Journal of Clinical Endocrinology and Metabolism, 2005;90(3):1311-1316.

Foster RG and Wuff K. The rhythm of rest and excess. Nature Reviews in Neuroscience, 2005;6:407-414.

Van Someren EJW and Riemersma-Van Der Lek RF. Live to the rhythm, slave to the rhythm.  Sleep Medicine Reviews, 2007;11:465-484.