The body's internal clock has a roughly 24-hour cycle that governs waking, sleeping and other physiological processes. It relies on daily environmental cues known as zeitgebers – the most important of which is exposure to morning light – to keep it in sync with the normal 24-hour day.

In people living in the polar regions, where the sky can stay dark for as long as six months at a stretch, the so-called circadian clock goes haywire because it doesn’t receive its natural cue – daily sunlight – to prompt them to wake or sleep, creating a sort of perpetual jet lag effect. As a result, people tend to go to bed roughly an hour later each day.

Similarly, teens who use smart devices such as computers to play games late into the night might find it increasingly difficult to nod off despite the lateness of the hour, as do those who work night shifts. “The body’s intrinsic biological clock regulates what time we wake up and when we feel sleepy. That clock is partly reset every day by external cues, the most important of which is morning light shining into our eyes,” said Asst Professor Leow Leong Chai​, Senior Consultant, Department of Respiratory and Critical Care Medicine​, and Director, Sleep Disorders Unit, Singapore General Hospital​ (SGH), a member of the SingHealth group.