In recent weeks, many of us have been asked to adopt a new, imperfect daily lifestyle – one that most knew was inevitable yet unsettling to say the least. Humans are often comically referred to as “creatures of habit” however, scientists have shown that loss of routine may have a subtle, but significant impact on our overall health.
Many key biological processes in our body occur at pre-determined intervals throughout the day. Scientists refer to our body’s internal daily routine as its circadian rhythm, from the Latin words circa (meaning “around”) and dies (meaning “day”). Whilst the master switch of our circadian rhythm is our sleep-wake cycle, the term can refer to any biological process that oscillates, or changes in a predictable pattern, throughout our day. This includes brain wave activity, hormone production, metabolism and our ability to fight infection, each of which use different changes in environment to trigger phases of their cycle.
When the circadian rhythm is compromised, the immune function is compromised. We now think that circadian health plays a role in making us more susceptible to disease or infection” – Dr Steven Lockley, Harvard, as reported in .
Interestingly, scientists have shown that our biology is made up of one central clock – which determines our sleep-wake cycle through light wave patterns – and several peripheral clocks, which respond to non-light clues such as food intake. As we wake, light enters our retina and stimulates specialised cells known as ganglion cells. These cells, along with other cell types, form the optic nerve which sends a message to a secondary bundle of nerves, known as the superchiasmatic nucleus (SCN). The SCN is located in an area of our brain known as the hypothalamus, which is responsible for relaying signals to ‘wake-up’ or ‘switching off’ other areas of the brain such as the pituitary and pineal gland(s). Whilst light signals stimulate release of some chemical signals, or hormones, such as cortisol, light simultaneously prevents release of another hormone, melatonin, from the pineal gland, allowing the cells in our body to synchronise with the external environment. As diurnal organisms, the opposite pattern occurs at night which is why melatonin is commonly known as the sleep hormone, and acts to lower body temperature and dictate the quality of our sleep.
Major alterations to the light-dark cycle or damage to the SCN have been shown to have detrimental effects on overall cognitive function, maintenance of body temperature, metabolism, and immunity. As explained by Micheal W. Young, during his Nobel Prize Speech in 2017, early studies in mice provided the first evidence that “a problem (may arise from) a multiplicity of clocks that can’t agree for several days”, with reference to why humans feel jetlag. Such findings have since led to the study of chronobiology, which aims to link circadian rhythms to disease states and provides insight into how the timing of treatment may enhance treatment-response for chronic conditions, such as cancer and metabolic disorders.
Internal desynchronisation that is such a problem for overall health” – Russell Foster, Professor of Circadian Neuroscience, University of Oxford (Podcast)
However, the importance of understanding chronobiology may also appear more subtle as scientists have found that differences in an individuals’ circadian rhythm, or chronotype, present a challenge in how best to advise those who have recently started working from home. As we live indoors and keep lights on at night, our inherent biological routines are less defined by the earth’s light/dark cycle compared to other species. However, this does not override the benefits of entraining our body to a daily schedule. Is it possible that subtle changes in the interval between two conditions – such as light vs. time (i.e. jetlag) or wake vs. activity (i.e. routine) – may alter the body’s natural sequence of hormone release or neural firing?
Early studies found that several tissues, other than the SCN, express circadian oscillations, however, scientists have found it hard to investigate the subtle regulation of peripheral time. Nevertheless, studies investigating the effect of shift work on metabolism suggest that regularly timed non-photic clues (i.e. food intake) are important for setting peripheral clocks. This seems less surprising when you consider that many circadian-driven changes inside our liver cells involve molecules that regulate the breakdown of carbohydrate (sugar) and lipids (fat). Since the 1960s, scientists have known that our body’s ability to breakdown glucose is best earlier in the day and one study  comments that short term (10-day) desynchronization may lead to increased blood pressure and increased glucose levels which, over time, may constitute increased risk of cardiovascular disease and prediabetic conditions as observed in shift workers.
Fortunately, the human body is highly adaptable, and it has been suggested that having several body clocks allows us to change our behaviour to suit daily schedules and work patterns without any immediate effect on our biological cues. However, particularly in times of stress and uncertainty, daily routine helps entrain a hard-wired ebb and flow in our body’s natural rhythms to prevent injury, metabolic disorders and heightened anxiety. This does not mean to say everyday must be filled with the exact same pattern of events, or that you must start work at 9 am sharp, but prioritising sleep, continuing daily exercise and eating your most substantial meal during daylight hours has a larger impact on our health than previously thought.
For further insight into how circadian rhythm affects our health, I’d recommend the TEDx talk by Emily Manoogian, a researcher in chronobiology, who asks us to pay attention to (our) body’s master clock.
Featured image: Nature Collection
The Naked Scientist – Clock this! The Science of the Circadian Rhythm
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