What would happen to our circadian rhythm if we lived in complete darkness? That is what Professor Nathaniel Kleitman of the University of Chicago and his assistant, Bruce Richardson, set out to answer in 1938. Equipped with enough food and water for six weeks, hospital bed, and devices that monitored their core body temperature and sleep-wake rhythms, they headed into Mammoth Cave, Kentucky. After over 30 days in total darkness, what they discovered (and has since been repeated with similar studies) is that in the absence of light, the circadian rhythm stretches out to somewhere in the range of 26-28 hours. The sun, in effect, winds our body clock back to synchronise each day.
How does light set the circadian rhythm?
The connection between humans and their environment is a relationship of interinfluence that goes back as far as life itself. While in recent times there has been an increasing focus on the ways humans impact the environment, we often forget the profound ways our environment has shaped our biology. Possibly one of the best examples that demonstrates this fact is the way we have evolved to use light to synchronise our own circadian rhythm with the Earths day/night cycle. The circadian rhythm is as old as life itself; every living organism on Earth displays an active and passive phase that correspond with the Earths day/night cycle. In humans, this endogenous rhythm dictates numerous physiological processes such as appetite, core body temperature and crucially wakefulness and sleep.
Being our primary sense, humans have somewhat intuitively understood the visual aspects of light. However, until recently, the role light plays in calibrating our circadian rhythm was largely unknown. This began to change with the discovery of a third receptor in our eye. Non-visual ganglion cells known as intrinsically photosensitive ganglion cells (ipRGC’s) are specially adapted to detect a specific spectrum and intensity of daylight and are most sensitive to the light of a blue sky (within the spectral range of 410nm – 550nm and a peak at 490nm). Due to their location in the bottom third of the retina they are optimally positioned to detect the light from the sky above us, rather than light shining directly into the eye and when stimulated these ipRGC’s send a signal to our master circadian pacemaker known as the superchiasmatic nucleus (SCN).
The SCN is the control center of the circadian rhythm. It responds to the input from the ipRGC’s by commanding the body with a careful balance of hormones and neurotransmitters that regulate our circadian rhythm. Of these hormones the most commonly referenced is melatonin as it is so fundamental to regulating sleep. During the night the concentration of melatonin in the bloodstream is 10 times higher than during the day, and as suppression begins it signals our body to prepare to wake up (Dubcovich, 2007). But melatonin does far more than just regulate sleep. It plays other important functions such as regulating fat cells, promoting growth hormones and maintaining the wellbeing of our immune systems (Srinvasan et al, 2005). The timing of melatonin production and suppression is crucial to maintaining our circadian rhythm and disruption of this timing can be very problematic to our sleep patterns.
Cortisol, another hormone administered by the SCN, also plays a key role in regulating our circadian rhythm. While cortisol is most commonly known for its role in supporting our bodies “fight-or-flight” response, it also promotes wakefulness. Following an inverse relationship to melatonin, cortisol levels reach their lowest just before we go to sleep. Whilst we sleep cortisol levels remain low, but gradually increase through the night. As we approach our wake time, we see a rapid increase in cortisol levels peaking approximately 30 minutes after waking up. This is known as the cortisol awakening response (CAR) and is fundamental to starting our circadian rhythm for the day (Fries et al, 2008). An hour after waking up cortisol levels are at pre-awake levels, then tapering off to its lowest point again around midnight.
Alongside melatonin and cortisol, the SCN controls another key neurotransmitter which aids our circadian rhythm called gamma-aminobutyric acid (GABA). This role of this inhibitory neurotransmitter is to lower the activity of neural cells in the brain and central nervous system as we prepare for sleep and does so through the initiation and maintenance of the various sleep stages (Lancel, 1998). Low GABA activity has been linked to disrupted sleep, increased anxiety and in severe cases insomnia. In a 2008 study of GABA levels in people, those with insomnia had GABA levels almost 30 percent lower than people without the sleeping disorder (Breus 2018).
As the sun rises, it begins to stimulate our ipRGC’s which in turn signal our body to begin suppressing melatonin and produce a burst of cortisol to activate our metabolic functions. As the sun sets and light disappears, our body lowers cortisol levels, begins secreting melatonin and increases the production of GABA. The cumulative result of these functions is a healthy cycle between wakefulness and sleep every day.
The problem is, in some sense, we are all living in Mammoth Cave. Okay, not literally Mammoth Cave, but, the artificially lit, multi-story environments in which we spend the overwhelming majority of our wakeful lives have more similarities with a cave than with to the natural environment we evolved to live in. The advent and proliferation of artificial light have caused the line between night and day be blur, as far as our biology is concerned. Worse yet, the light typically used indoors during the day is almost devoid of the wavelengths our ipRGC’s are most sensitive too. And just like in Mammoth Cave, in the absence of light to set our circadian rhythm, our body clocks begin to elongate. In practice, this leads to night owl tendencies; people staying up later into the night or struggling to fall asleep at night. And the same technological advancements that are failing to meet our circadian needs during the day have made it easier than ever before to occupy the night. Only a few hundred years ago the mere idea of working after sunset was met with some very practical challenges, but now it’s as simple as flicking the light on. Regardless of when you feel like going to sleep at night, the sun still rises at the same time the next morning. Work or school still start at the same time. This imbalance between when we want to go to sleep and wake up and when we need to go to sleep and wake up is a phenomenon referred to as social jetlag. And the type of chronic sleep deprivation social jetlag causes – a few hours shaved off each night – has been linked to a host serious health conditions, so it’s a problem worth addressing.