It's very likely that you are reading this text in an interior space with the lights on. For most people, modern living entails spending most of the day in closed rooms, bathed in a sum of artificial and natural lights. Yet while artificial light has afforded mankind incalculable possibilities, it has also caused some confusion in our bodies, which have evolved for thousands of years to respond to the stimuli of sunlight in the day and darkness at night. This responsiveness to natural light is called the circadian rhythm or cycle, and describes the 24-hour biological cycle of almost all living beings. Circadian rhythms are primarily influenced by light reception, but temperature and other stimuli also play a role in the process.

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