Our internal “biological” or circadian clock regulates many aspects of our physiological functions and behavior, including preferred sleep times, times of peak mental performance and the coordination of physiological processes, such as when blood pressure is highest or when the heart is most efficient.
A person’s chronotype, otherwise known as circadian preference, describes their inclination toward an earlier or later sleep schedule. Although there is significant genetic variation throughout the human population, people are often categorized into “morning people” or “larks” who prefer going to sleep and waking earlier, “evening people” or “night owls” who prefer a later bed- and rising time, as well as “intermediates” who are in between these two extremes. Age, gender, external light, and room temperature can all play a role; however, genetic variation is also an important contributor. Several twin and family studies have suggested that genetic factors explain up to 50% of the population variability in circadian timing. Wherever you fall on the morning-evening preference continuum is influenced by many genes, which is what defines this trait as polygenic. This means that whether you are an “early bird” or a “night owl” is not determined by a singular gene, but rather the interaction of several, along with external environmental and lifestyle factors.
Our biological clocks are composed of specific sets of proteins, which are found in cells and tissues throughout the body. Each cell in the human body maintains its own circadian rhythm, and the “master clock” found in the suprachiasmatic nucleus (SCN) of the hypothalamus in the brain maintains synchronization of these cellular clocks. For more context, the hypothalamus’ primary function is to keep the body in stable, constant condition (i.e. homeostasis); it responds to a variety of signals from the internal and external environment by putting changes in place to correct any imbalances.