Human circadian variations

Abstract

Circadian rhythms, present in most phyla across life, are biological oscillations occurring on a daily cycle. Since the discovery of their molecular foundations in model organisms, many inputs that modify this tightly controlled system in humans have been identified. Polygenic variations and environmental factors influence each person's circadian rhythm, contributing to the trait known as chronotype, which manifests as the degree of morning or evening preference in an individual. Despite normal variation in chronotype, much of society operates on a "one size fits all" schedule that can be difficult to adjust to, especially for certain individuals whose endogenous circadian phase is extremely advanced or delayed. This is a public health concern, as phase misalignment in humans is associated with a number of adverse health outcomes. Additionally, modern technology (such as electric lights and computer, tablet, and phone screens that emit blue light) and lifestyles (such as shift or irregular work schedules) are disrupting circadian consistency in an increasing number of people. Though medical and lifestyle interventions can alleviate some of these issues, growing research on endogenous circadian variability and sensitivity suggests that broader social changes may be necessary to minimize the impact of circadian misalignment on health.

Figure 1. Conventional sleep period timing compared with delayed sleep phase, advanced sleep phase, and natural short sleep.

Note that the “conventional sleep period” can be very variable. The timing of “conventional sleep” in this figure (10 pm to 7 am) is simply one example close to the population average. Adapted with permission from Neuropsychopharmacology (127).

Figure 2. Contributors to timing and duration of sleep.

Shown is a conceptual framework to demonstrate the multiple factors impacting the timing and duration of sleep. Genetic factors interact with environment, including light, social schedules, work and school obligations, and substances (alcohol, caffeine, medications). The result may be sleep duration and timing that differ from those suggested by genetics.