The effects of a modern lifestyle on circadian rhythms and on human health
By Farhana Alam
In 1729, French astronomer, Jean-Jacques d’Ortous de Mairan, placed a mimosa plant into a dark cupboard and observed that the raising of its leaves at dawn, and their drooping at dusk, continued despite the lack of a light stimulus. The plant appeared to have its own, internal 24-hour cycle. It was another 230 years before the word, “circadian” (meaning ‘around a day’), was coined. We now know that circadian rhythms are present across the domains of life: in bacteria, fungi, algae, plants and animals.
In mammals, the main circadian pacemaker is in the suprachiasmatic nucleus (SCN); a tiny brain region located above the junction of the two optic nerves. The SCN is driven by changes in light intensity, detected by nerve cells in the retina, and it regulates subsidiary clocks, present in almost all cells of the body. The 2017 Nobel Prize in Physiology or Medicine was awarded to Jeffrey Hall, Michael Rosbash and Michael Young, for their identification of ‘clock’ genes that drive the cellular process in these subsidiary clocks. The period gene encodes the PER protein, which is produced at night but degrades during the day. The timeless gene encodes the TIM protein, which is required for entry of both proteins into cell nuclei, where they inhibit the period gene in a classic negative feedback loop. Circadian rhythms influence a plethora of physiological reactions. The inflammatory response is greater during the night, as a body at rest is better able to fight off infection; however, this means that the time of day that a person has surgery or takes a drug/medication will affect its outcome. Car accidents occur at a disproportionately higher rate at around 3am, due to low alertness, and workplace accidents are also more likely to occur at night. In 2011, the World Health Organisation produced a list of health risks associated with shift work. These include gastrointestinal disorders, cardiovascular conditions, breast and colorectal cancer, preterm delivery and psychological health problems.
Sleep and circadian rhythm disruption (SCRD) has been reported in over 80% of depression or schizophrenia patients, and is also linked to neurodegenerative diseases, such as Alzheimer’s and Parkinson’s. However, the direction of causation in these cases is unclear. The most well-known disruption to circadian rhythms is jet lag, caused by mismatching of internal and external clocks. In fact, patients with cardiac disorders are at greater risk of heart attack not just after long flights, but also in the week following clocks going forward or backward.
Teenagers are a special case; between ages 12–20, humans have a delayed body clock, resulting in later sleeping/waking times. Adults tend to be fully alert by 10am, but most teenagers are not fully alert until 12 noon, with 10–20% not fully alert until 2pm. Unfortunately, this means that changing the timings of the school day to benefit students may result in poorer quality teaching from tired teachers; a quandary!
Our 24/7 society is making SCRD more prevalent. The vast majority of the population awakes well before their circadian clock would naturally wake them, whilst radiant screens and bright LED streetlights prevent early nights. These disruptions have coalesced into what is termed ‘social jet lag’ (SJL), which is defined as the difference in sleeping hours between work days and free days and is exacerbated by the pandemic use of alarm clocks. SJL is now widespread and is linked to increased risks of developing obesity (33% increased risk for every hour of SJL), Alzheimer’s and cancers. The best way of combatting SJL is to stick to a routine of waking, eating and sleeping like clockwork, i.e. tune in to your own rhythm and try not to step out of time!
From Issue 16