IELTS Academic Reading Practice Test

Nearly every living organism on Earth — from single-celled cyanobacteria to complex mammals — operates according to an internal biological clock that runs on a cycle of approximately 24 hours. These rhythms, known as circadian rhythms (from the Latin circa diem, meaning 'about a day'), govern an extraordinary range of physiological processes: body temperature, hormone secretion, immune function, digestion, and the timing of sleep itself. Far from being a simple on/off mechanism for wakefulness, the circadian system is a sophisticated network that coordinates cellular activity across the entire body, anticipating environmental changes before they occur rather than simply reacting to them.

Although biological timekeeping has been observed for centuries — even the ancient Greek physician Androsthenes noted daily leaf movements in plants — the scientific study of circadian rhythms accelerated dramatically in the twentieth century. American biologist Franz Halberg formally coined the term 'circadian' in 1959 and pioneered the field of chronobiology, the study of time-dependent biological processes. A pivotal advance came in 1971 when Seymour Benzer and Ronald Konopka identified the first gene governing circadian function in the fruit fly Drosophila melanogaster, opening the door to understanding the molecular machinery behind the clock. The field received its highest recognition in 2017 when Jeffrey Hall, Michael Rosbash, and Michael Young were awarded the Nobel Prize in Physiology or Medicine for elucidating the molecular mechanisms of circadian rhythms.

In mammals, the master circadian clock resides in a small region of the brain called the suprachiasmatic nucleus (SCN), located within the hypothalamus, just above the point where the optic nerves cross. The SCN receives light input directly from photosensitive cells in the retina and uses this information to synchronise internal time with the external environment — a process called entrainment. As darkness falls and light input diminishes, the SCN signals the pineal gland to release melatonin, a hormone that promotes sleep onset and is strongly suppressed by light. This daily hormonal rhythm acts as a broadcast signal, communicating time of day to tissues and organs throughout the body.

The significance of a well-functioning circadian system becomes particularly evident when it is compromised. Epidemiological research has consistently shown that people who work night shifts or rotating schedules — an estimated 20 percent of the workforce in industrialised nations — face significantly elevated risks of a range of chronic conditions. Studies have linked circadian disruption to higher rates of type 2 diabetes, cardiovascular disease, obesity, depression, and certain cancers. The International Agency for Research on Cancer classified night shift work as a probable human carcinogen as early as 2007. These findings suggest that misalignment between internal biological time and external social demands is not merely an inconvenience but a measurable threat to long-term health.

Emerging from this body of research is the relatively new field of chronotherapy — the practice of timing medical treatment to align with the body's circadian rhythms in order to maximise therapeutic benefit and minimise adverse effects. Clinical trials have demonstrated that the timing of chemotherapy infusions can significantly influence both toxicity and treatment outcome; administering certain cancer drugs during periods when tumour cells are most vulnerable, while healthy cells are at their most resistant, has improved patient survival rates in a number of trials. The principle extends further: the timing of blood pressure medication, anaesthesia, and even surgical procedures has been found to affect outcomes in ways that were largely overlooked before chronobiology entered clinical practice.

The growing mismatch between human biology and modern social patterns constitutes an expanding public health concern. Electric lighting, first widely adopted in the early twentieth century, extended the waking day far beyond what our evolved biology was shaped to accommodate; the proliferation of LED screens — which emit disproportionate amounts of short-wavelength, high-energy blue light — has compounded this problem further. Chronobiologists have coined the term 'social jetlag' to describe the discrepancy between an individual's natural sleep timing and the timing imposed by work, school, or social obligations. For many people, this amounts to the physiological equivalent of crossing one or two time zones every working week, with cumulative consequences for cognition, metabolism, and cardiovascular health.

Researchers and policymakers are beginning to grapple seriously with the implications of circadian science for public life. Evidence that adolescents have chronotypes naturally shifted toward later sleep and wake times prompted several countries — including the United States, the United Kingdom, and Australia — to trial later school start times, with early results suggesting improvements in student attendance, academic performance, and mental health outcomes. In medicine, the concept of circadian profiling — establishing an individual's biological clock phase to optimise treatment timing — is moving from laboratory investigation toward clinical implementation. What began as an observation of leaf movements in an ancient garden has evolved, through centuries of scientific enquiry, into a discipline with the potential to reshape how healthcare, education, and urban life are organised around the fundamental rhythms of human biology.