Nearly all living organisms, from bacteria to humans, possess an internal biological clock that regulates a roughly 24-hour cycle of physiological and behavioural changes, known as the circadian rhythm. In humans, this internal clock is primarily governed by a small cluster of neurons located in a region of the brain called the suprachiasmatic nucleus, which receives direct input from specialised cells in the eye that detect light levels. Exposure to light, particularly in the morning, helps synchronise this internal clock with the external day-night cycle, a process that explains why exposure to bright artificial light late at night, such as from smartphone screens, can disrupt sleep patterns by confusing the body's sense of time. Circadian rhythms influence far more than just sleep; they also regulate body temperature, hormone release, digestive activity, and even cognitive performance, which tends to peak and dip at fairly predictable times throughout the day for most individuals. Disruptions to circadian rhythms, whether caused by shift work, jet lag, or chronic irregular sleep schedules, have been linked to a range of negative health outcomes, including increased risk of metabolic disorders, cardiovascular disease, and impaired immune function. Researchers have also identified genetic variation in circadian rhythm timing among individuals, which partly explains why some people naturally function best in the early morning while others feel most alert and productive later in the day, a distinction commonly referred to as one's chronotype.