The term circadian is derived from Latin to mean “about a day.” Organisms ranging from single-cell organisms and plants to sea slugs and humans demonstrate circadian rhythms near twenty-hour patterns. In humans, numerous physiological and behavioral processes demonstrate circadian rhythms but the sleep-wake rhythm is one of the most important and observable cyclical rhythms. The mean duration of a human’s endogenous circadian clock is approximately twenty-four hours and fifteen minutes and light from the sun methodically “resets” us to a twenty-four cycle on a daily basis. Every morning, the sun “rises” and every evening it “sets.” This has been true for the last 4.5 billion years of the Earth’s existence. Evolution uses this reliable signal to synchronize our “master clock” to the daily orbital mechanics of the planet spinning on its axis resulting in regular phases of dark and night. The Sun serves as a zeitgeber – ‘time giver.’
Circadian rhythms are coordinated by the area of the brain that incorporates and processes the light and day signal. This neural cluster, called the suprachiasmatic nucleus (SCN), consists of 20,000 neurons and is located at the crossing point of the optic nerves as they exit the eye. This proximity enables it to sample the light signal being sent from the eye to the visual cortex located at the posterior of the brain (occipital lobe). In response to this signal, the SCN facilitates secretion of the hormone melatonin from the pineal gland. This, in turn, informs the body that it is dark and helps regulate the timing of when sleep occurs. Melatonin concentrations peak in the middle of the night and decrease as sunlight enters the brain through the eyes. Although there is no variation in the twenty-four-hour pattern, the respective timing of the peak and trough of melatonin concentrations vary from one individual to the next. This variation accounts for the spectrum of chronotypes, more commonly identified as ‘night owls’ and ‘morning larks.’
Circadian misalignment occurs when evening chronotypes must wake up early for day shift work, and morning chronotypes night shift-work. This disconnect in chronotype and timing imposed by work obligations, also known as social jetlag, can lead to sleep debt, poor work performance, and the development of chronic health conditions. In the last century, with the twenty-four hours needs of a modern industrial economy, shift work schedule has become increasingly more common. Currently, at least 25% of the workforce is a shift worker. This trend has inevitably crept into health care as many physicians and nurses also work a shift schedule. Most shift workers experience circadian disruption and sleep curtailment and perform poorer on memory tasks with a greater incidence of gastric ulcers and depressive symptoms. Evidence also suggests that shift workers are more prone to cardiovascular disease, diabetes, obesity, cancer, and hyperlipidemia. In fact, the World Health Organization (WHO) lists shift work as a possible carcinogen. Preliminary evidence also supports the notion that shift workers have improved sleep duration, quality, and decrease in social jetlag when the shift time correlates with the worker’s chronotype.
Despite this variation around sleep and wake times, humans are essentially diurnal. We are alert, active, and awake during daylight hours and asleep at night. The SCN activates many brain and body mechanisms during daylight hours to keep you awake and alert and conversely, deactivates these processes during the night. In effect, removing the alerting influence. Dopamine-responsive centers for impulsivity and reward are hyperactive in sleep-deprived individuals resulting in a higher likelihood of emotionally volatile and rash decision making. This has played out in a myriad of tragedies and statistics in our modern twenty-four-hour world. The industrial accidents at Bhopal, Chernobyl, and Three Mile Island all occurred between midnight and 4am. Field studies show that between 3am to 5am, workers are slowest to answer a telephone, slowest to respond to a warning signal, and most apt to read a meter wrong. Truck drivers are much more likely to have an accident at 5am than during regular daytime hours. These late night hours are often referred to as the zombie zone by shift workers. Whereas reducing non-essential night shift work is an obvious solution, fields such as emergency response, healthcare, and protective services require twenty-four-hour coverage.
The implications of these findings seem to be underappreciated but do impact all levels of healthcare from patients and physicians to health insurers and providers. For example, should chronotypes be considered for night vs day shift workers? Are circadian misalignment and social jetlag among the factors leading to the epidemic of physician burnout? Can physicians consistently make emotionally rational decisions and judgments during the zombie zone? Should there be a differential gold standard for decision making at night? What is the cost to patients who are seen in Emergency Departments in the middle of the night? Should physician reimbursement and charge be different (higher or lower?) for night shifts? Experientially, I can personally vouch to struggling with food impulses, metabolic syndrome, weight gain, and decision making over the last decade of night shifts. From my perspective, robust cognitive support in the form of augmented intelligence is a multi-level solution when it comes to improving decision making (next essay).