Chronobiology is the scientific study of biological rhythms and how living organisms adapt their functions and behavior to the natural cycles of day and night. Human beings possess an internal biological clock that regulates sleep, wakefulness, hormone secretion, body temperature, metabolism, digestion and many other physiological processes. This internal timing system is closely linked to the 24-hour light–dark cycle and is known as the circadian rhythm. In modern society, however, work schedules, school timings, social activities, and excessive exposure to artificial light conflict with these natural rhythms. One important consequence of this conflict is “social jetlag.”
The circadian rhythm is primarily controlled by a group of cells in the brain called the suprachiasmatic nucleus (SCN), located in the hypothalamus. Light entering the eyes provides the major environmental signal that synchronizes this biological clock. In the evening, darkness promotes the production of melatonin, a hormone that facilitates sleep, while exposure to light in the morning helps suppress melatonin and promotes alertness. Ideally, sleep and waking should occur in harmony with this internal clock.
However, many people, especially in bigger cities, do not follow their natural sleep–wake cycle during the working or academic week. They may wake early on weekdays because of school or employment commitments and then sleep considerably later on weekends. This difference between an individual’s preferred biological schedule and their socially imposed schedule is called social jetlag. It resembles the experience of travelling across time zones, although no physical travel is involved.
Social jetlag is particularly common among adolescents and young adults because their biological clocks tend to shift toward later sleep and wake times during puberty. Early school and university schedules may therefore force them to wake before their bodies are biologically prepared to do so. Adults may also experience social jetlag because of long working hours, shift work, commuting, late-night entertainment and excessive use of smartphones and computers.
Persistent disruption of circadian rhythms may have important consequences for physical and mental health. Insufficient or irregular sleep can impair attention, memory, learning, decision-making and emotional regulation. People experiencing substantial social jetlag may feel tired during the day and may rely on caffeine or other stimulants to maintain alertness. Chronic circadian disruption has also been associated with metabolic disturbances, weight gain, impaired glucose regulation, cardiovascular problems and mood disorders. Although social jetlag itself is not necessarily the sole cause of these conditions, it may contribute to unhealthy patterns of sleep and behavior.
The modern digital environment has intensified the problem. Exposure to bright artificial light, particularly from smartphones, tablets, computers and televisions during the evening, can delay melatonin secretion and make it harder to fall asleep. Late-night social media use and online entertainment can further encourage people to postpone bedtime. Consequently, an individual may experience a mismatch between biological sleep needs and socially determined schedules.
Reducing social jetlag requires greater awareness of the importance of sleep and biological timing. Maintaining a relatively consistent bedtime and wake-up time throughout the week is one of the simplest strategies. Morning exposure to natural daylight can help synchronize the circadian clock, while reducing bright light and screen exposure before bedtime may support healthy sleep. Schools and workplaces can also contribute by considering more flexible schedules where feasible, particularly for adolescents and shift workers.
The Author is BDS, M Phil Physiology, Sr. Instructor, Dept of Biological & Biomedical Sciences, Aga Khan University, Karachi