IELTS Reading: True / False / Not Given

Almost every organism on Earth — from the simplest bacterium to the most complex primate — possesses an internal biological clock that generates rhythms of approximately 24 hours, even in the absence of external time cues such as light and darkness. These rhythms are known as circadian rhythms, from the Latin circa diem, meaning 'approximately a day'. In humans, the master clock resides in the suprachiasmatic nucleus (SCN), a tiny cluster of approximately 20,000 neurons located in the hypothalamus of the brain. The SCN coordinates rhythms across the body by sending signals — chemical and neural — to peripheral clocks found in virtually every tissue and organ, from the liver to the heart to the skin. Even individual cells in isolation can maintain circadian oscillations, cycling through gene activation and suppression in a predictable 24-hour pattern. The core molecular mechanism of the clock was elucidated largely through research on the fruit fly Drosophila. Scientists identified a set of 'clock genes', including Period (per), Timeless (tim), and CLOCK (Clk), whose protein products form interlocking feedback loops. These proteins accumulate slowly over the day, eventually suppressing their own production, and then degrade during the night, allowing the cycle to restart. This delayed negative feedback loop creates an oscillation of approximately 24 hours. Light is the primary zeitgeber — a German word meaning 'time giver' — that synchronises the internal clock with the external environment. Specialised photoreceptive cells in the retina, distinct from the rods and cones used for vision, detect specifically blue-wavelength light and transmit signals directly to the SCN via the retinohypothalamic tract. Even brief pulses of light during the biological night can shift the phase of the circadian clock, which explains why exposure to the blue-light emitted by screens late in the evening can disrupt sleep onset. Disruption of circadian timing — through shift work, transatlantic travel, or irregular sleep schedules — is associated with increased risk of metabolic disorders, cardiovascular disease, and certain cancers, underscoring the fundamental importance of this timekeeping system to health.