Lisa Heschong,
DLA member
31 October 2024
Celebrating Dark Nights
We are lucky that our planet rotates. During the day, the sun’s intense radiation warms the earth and brightens our beautiful blue atmosphere. At night, in the sun’s dark shadow, the earth cools, sending its heat back out into infinite space. Because of its 24-hour rotation, no one face of the planet gets burned by unrelenting radiation, or is forever deprived of the sun’s life-giving energy.
However, the extreme physical contrast between these two conditions—very bright days, and very dark nights—created a supreme challenge for all forms of life on the surface of the planet. How to survive both extremes? Complicating matters, there are seasonal variations in solar intensity and the length of day and night due to the tilt of the planet relative to its elliptical path around the sun. There are also significant variations in the very darkness of night due to the phases of the moon, and even little ‘wobbles’ in both the Earth’s and moon’s orbits, interjecting subtle variations over time. To survive year to year, life forms needed to predict: will my surroundings be getting progressively brighter or darker, warmer or colder? Will my food supply be increasing or disappearing? Do I need to get ready to hibernate or rather get busy gathering enough food and energy to reproduce?
Life on Earth solved this challenge with the evolution of biological clocks, finely tuned via exposure to the changing ratios of bright days to dark nights. Scientists have now identified ‘clock genes’ which maintain approximately 24-hour (circadian) rhythms in every cell in our body, and similarly in every other plant and animal studied so far. These clock genes are so ancient that they are even found in cyanobacteria, also known as blue-green algae, thought to be one of the oldest life forms on Earth.
Figure 1. KaiC, the clock protein that regulates circadian rhythms in cyanobacteria
Whether a creature is diurnal or nocturnal, whether a plant flowers during short or long days, exposure to daily rhythms of light and dark not only orchestrate patterns of rest and activity, repair and reproduction for each species, but also help to keep whole ecosystems in synchrony with each other. Sunflowers which track the sun during the day, automatically rewind during the night to face east, long before the sun rises. Later, in the cool early morning hours, pollinating bees favor those warmer sunlit flowers. Birds know when to nest in the spring and when to migrate in the fall by sensing changes in day length and temperature, which predict the coming season of abundance or scarcity. Long before birds begin to build nests in the spring their bodies are already automatically increasing the size of their reproductive organs, and after fledging their chicks, their bodies automatically start increasing their stores of body fat in preparation for long migrations in the fall.
Figure 2. Exposure to the changing ratio of dark nights to bright days drive birds’ seasonal behaviors
While it may be easy for us humans to observe how planetary rhythms dominate the lives of plants and animals, it is often far more challenging for us to acknowledge the importance of such planetary rhythms in our own lives. Over the past 100 years or so, we have dramatically changed the patterns of light that we live with. In pre-industrial societies humans typically spent most of their days outdoors active in bright daylight, and then retreated indoors at night, with only dim firelight or candlelight for illumination. Now, with the widespread adoption of electric lighting both indoors and out, along with 24/7 lifestyles, we have steadily eroded the biological difference between day and night. The amplitude of our circadian signals is diminishing. While human beings are amazingly adaptable, thriving in a vast range of environments across the globe, evidence is mounting that pervasive attenuation of our biological rhythms is resulting in long term disruption of human health.
We now understand that a vivid two-stroke daily rhythm of ‘bright/dark, bright/dark’ helps to keep our bodily systems in sync, both with external planetary seasons, but also internally, by coordinating the myriad of peripheral clocks operating in our cells and organs. Studies have shown that exposure to ample daylight helps us sleep better at night, while full nighttime darkness improves both sleep quality and our performance the next day. As diurnal animals, our bodies maintain two distinct metabolic systems: one supporting optimum activity during daylight hours and the other enabling mental and physiological restoration during darkness at night. This elegant system is truly a biological marvel.
So, join with me in celebrating the coming of darkness at night, just as we look forward to the dawning of each day. For without darkness, there is no dawn…
More on this topic is available in the online keynote “Day and Night, Life and Light”
from Lisa Heschong at DarkSky International’s 2022 Annual Conference
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