We feel our bodies ebb and flow on a daily basis, the rhythm of our lives dictated by patterns and repetitions that are monotonous in their consistency. Our biology is constantly cycling: through stages of birth and rebirth, growth and regrowth; through cells, which constantly grow, divide and grow again to prop up every functional aspect of our physiology. Our development throughout our life is dictated by molecular mechanisms that feedforward and then feedback onto themselves to facilitate the transition between our first day and our last. Although human biology is often reduced to a spatial map of physiology, it is important to see this map not just in space but also throughout time. Our body cycles matter.

We are most familiar with the diurnal cycle, the full rotation of the earth with respect to the sun, resulting in night and day. If we consider our body to be a 24-hour clock, our activity and alertness, hormone levels, body temperature, immune functions and more fluctuate enormously throughout the day. These biochemical, physiological, and behavioural cycles are dictated by the suprachiasmatic nucleus (SCN), a tiny portion of our brain in the hypothalamus located directly above the optic chiasm of our eyes that acts like a pacemaker. Each neuron in the SCN is its own functional clock, normalized to its neighbours by a constant stream of light information fed to the SCN by photosensitive cells in our eyes. When the SCN was first discovered, the complex interplay that kept us on schedule was thought to be limited to the brain, but it has become clear that this central clock coordinates what are known as molecular oscillators (a cell or group of cells that act as a clock and cycles every 24 hours) and time-observing genes throughout the rest of the body.

The researcher and proclaimed father of sleep research, Nathaniel Kleitman, and his colleague, Bruce Richardson from University of Chicago spent thirty-two days in a cave in 1938 to explore the question “does my sleep-wake cycle continue, even in the dark?” The answer was yes. These first temporal-isolation experiments found that our natural circadian rhythms persist despite isolation, serving as powerful evidence for an endogenous clock inside our brains.

In one particularly impressive study, conducted by French researcher Michael Siffre, who stayed in a cave for 63 days found that his sleep-wake cycle remained pretty constant despite the temporal-isolation. Interestingly, his sleep-wake cycle shifted relative to the actual alternation between night and day of the outside world once placed in isolation but once the experiments were over only took a few days to readjust his cycle to the external light cues. In this experiment, Siffre was exposed to natural variations in light and other external cues that characterize time prior to entering isolation. Following this control period, he was then isolated from these cues and his body was left to its own endogenous rhythm. He continued to display a consistent sleep-wake cycle, though it was observed that he lost a ‘biological day’ because his rhythm lengthened to about 25.5 hours. It was also observed that the Siffre’s cycle shifted without consideration for what time of day it was, initially waking up regularly 8AM but eventually having the cycle shift such that he would wake up at midnight and feel perfectly in sync. What was remarkable is how quickly his systems readjusted to external cues of night and day when he exited isolation, returning to normal in just a few days.

Science has made it clear that our body runs on a 24-hour schedule. Our genes have evolved to account for the rotation of the earth, the amount of light, and the change in temperature and research has shown that light is the primary synchronizer of our rhythms. Without it, our master pacemaker becomes misaligned, which in turns skews the peripheral clocks further down our body hierarchy. The effect of the shift of these delicate biological clocks can be observed most acutely in the phenomena of jet lag, whereby skipping over a few time zones requires a long period of readjustment.

What are we to do then in a world that is increasingly brightened by artificial light? In an artificially lit and industrialized world, our biological clocks no longer receive strong synchronizing signals. Since Thomas Edison first burned an incandescent light bulb in 1879, light has flourished; it has been found that the glow of Los Angeles can be seen by a plane more than 200 miles away. Modern life has scattered our sleep-wake cycles around the clock, and our circadian desynchrony is reinforced by lifestyles that keep us indoors, removed from natural light and exposed to artificial light even after sunset.

The effect of unending brightness on our bodies has not yet been well defined, but it’s clear that there is epistemological evidence to suggest there will be both short term and long term effects on our well-being. When people operate on schedules out of alignment with their internal rhythms, they are exposed to sleep restrictions and disruptions leading to a general decline across the vast majority of cognitive domains. Things like alertness, attention and working memory are deeply impaired by this sleep debt.

Beyond sleep disorders, a shift of your circadian clock can have profound effects on the rest of your physiology. In a study conducted by Mukherji et al. in 2015, it was documented that altering the circadian rhythm of mice by altering their feeding schedule led to negative metabolic responses (increased blood pressure, blood sugar, fat accumulation) because it threw off the mice’s peripheral circadian clock. This caused a non-expression of important receptors in the SCN that prevented the SCN from shifting the new feeding schedule, ‘breaking’ the peripheral body clocks. This continued to produce more profound effects: first, the shifted schedule ‘broke’ the peripheral clocks which reduced a reduction in insulin secretion which results in what we often seen in diabetes, high blood sugar, that gives way to a host of other problems. It equally causes an overproduction of the stress hormone, corticosterone, that eventually leads to problems in muscles and the heart. In other studies, there is increasing amount of evidence that artificial light exposure and alterations in circadian rhythms can contribute to the development of depression and mood disorders, as well as elevated risk of breast cancer, cardiovascular disease, and obesity. Though the role of the disruption of our rhythms in these health problems isn’t clear, it’s important to note that our body cycles play a key part in exacerbating the risk of developing these problems.

The idea of our body as a clock is not just a nice metaphor but a relatively accurate one that emphasizes the need to tune our watches from time to time. Artificially lit lifestyles are not inherently bad — without it we would most likely lead boring lives from 6AM to 6PM (or 4PM for us unlucky Canadians in December) — but in a world where there is not enough hours in the day and efficiency is the ultimate priority, taking the time to tune into the cyclic nature of our lives is far from trivial. The repercussions for not paying attention could unfortunately be more costly for our overall health than most of us care to realize.