Our circadian cycle
Light is essential for our lives, not only for our vision, but also for our life cycle. How our body functions depends more or less on the time of the day; our biological clock (or circadian cycle) determines how we feel and the state of our body. In recent decades, scientists have discovered our brain has light-sensitive master pacemaker neurons that are connected to a network of peripheral “clocks” located in individual cells in different parts of our body. The master clock controls our sleep/wake cycle, learning, reward and other behaviors. The peripheral clocks are synchronized to function at different times: for digestion, respiration, body temperature, hormones, immune or other functions in our body and interactions with our environment.
Disruption of our circadian cycle has consequences for our health and mental well being, causing our peripheral clocks to become uncoupled and desynchronized from the master pacemaker. This can happen as we age, do shift work with irregular schedules, undertake jet travel to different time zones or overextend our waking hours at night under artificial day light. Lack of sleep disrupts our hormone system so that we are less alert, have a foggy brain and lose motor skills. Poor sleep can raise our blood pressure and affect our appetite. When people try to make up for the loss of night sleep during the day, they sleep fewer hours and less deeply. This is because exposure to sunlight prevents the brain from producing melatonin and other natural soporifics.
Not surprisingly, longtime night shift workers suffer from obesity and cardiovascular disease at rates up to double that of daytime workers. Similar results have been observed in mice—mice that ate during the light phase of the circadian cycle were thinner than those restricted to eating during the dark phase, even though they ate the same amounts of a high fat diet. In experiments simulating the effect of jet lag, mice lived under conditions where lights were switched on and off at varying times each week. Despite a healthy diet, these mice gained weight and developed fatty liver disease, which progressed to fibrosis and, in some cases, liver cancer.
Scientists have increasing appreciation that almost all forms of life follow a circadian cycle. It is observed in one-celled cyanobacteria, in plants and in the animal kingdom, whether the organisms march to a diurnal clock like man or are nocturnal like bats. Man seems to be the only living being that consciously disrupts his circadian cycle by his lifestyle. Since the stability of our mental and physiological health is maintained by a steady circadian cycle, their chronic disruptions may contribute to the development of disease states such as obesity, diabetes, cancer, and cardiovascular, neurological and gastrointestinal disorders. Our biological clocks are attuned to our environment and appear to integrate our cellular control to the operation of our whole body, making us fit or diseased. To maintain our health, we should keep our activities in sync with our circadian cycle.