Millbrook: IELTS General Training Reading practice test

Almost every process in the human body runs on a daily cycle, and the cycle is not a response to the sun but a rhythm generated internally, by a cluster of a few thousand cells in the base of the brain. Left in constant dim light, a person keeps a rhythm of about twenty-four hours and a little more. What the sun does is reset it each morning, through a class of cell in the eye that is not used for vision at all and responds most strongly to blue light. That rhythm governs core temperature, alertness, the timing of several hormones and much of digestion, and all of these have a low point in the small hours.

A very large number of people are nonetheless required to be awake and competent at exactly that point. Depending on how the question is asked, somewhere around a fifth of employees in industrialised countries work outside ordinary daytime hours, and the proportion has not fallen. Hospitals, policing, fire services, air traffic control, long-distance transport, continuous-process manufacturing and the warehouses that supply next-day delivery all require someone to be at work at four in the morning, and no rearrangement of the economy is going to change that.

The consequences show up first in performance. Reaction times lengthen, attention wanders and the willingness to check a decision falls away, with the worst of it between two and six in the morning. Several of the twentieth century's best-known industrial accidents began during those hours, which proves nothing on its own but is consistent with a great deal of laboratory work showing that sustained wakefulness degrades judgement in ways the person concerned cannot detect. It is worth adding that for many shift workers the most dangerous part of the job is not the job. It is the drive home afterwards.

The second consequence is sleep itself. Sleep taken during the day is shorter and more broken than sleep taken at night, and not mainly because of noise and light, though both matter. It is shorter because it is being attempted at the point in the cycle when the body is preparing for activity, and a rising core temperature ends sleep whether or not the sleeper has had enough. Across a range of studies, shift workers average between one and four hours less sleep in each twenty-four than comparable day workers, and the deficit does not accumulate quietly: it is repaid, imperfectly, on rest days.

Beyond fatigue lies a literature on long-term health that has to be read carefully. Night and rotating shift work is associated with higher rates of cardiovascular disease, of disorders of blood sugar and body weight, and, more contentiously, of certain cancers. Almost all of this evidence is observational, and shift work is not randomly distributed: it is concentrated in occupations that are lower paid, more physically demanding and more likely to involve smoking and irregular meals, all of which affect the same outcomes. An international review classified night work involving circadian disruption as a probable cause of cancer in humans, and a later review kept the classification while describing the human evidence as limited. Both statements are accurate and they sit together uncomfortably.

What can actually be done divides into things that are known to help and things that are merely popular. Rotas that move forwards through the day, from mornings to afternoons to nights, are tolerated better than rotas that move backwards, because the human rhythm lengthens more easily than it shortens. A short nap taken before or during a night shift is the best evidenced single measure available and improves alertness for several hours afterwards. Limiting the number of consecutive nights worked, controlling light exposure at the end of a shift, and moving the main meal of the shift earlier all have supporting evidence, though none is dramatic.

Against that, a good deal of common advice does less than it promises. General instructions to keep a regular routine are of little use to somebody whose routine is set by a rota. Melatonin shifts the body clock reliably, but only if taken at the right point in the cycle, and taken at the wrong point it does nothing or makes matters worse. And the most persistent assumption of all, that people get used to it, is largely false: most permanent night workers never fully adjust, because they revert to daytime hours on their days off, which is both entirely reasonable and precisely what prevents the rhythm from settling.