Sleep, as Shakespeare noted in Macbeth, is the chief nourisher in life’s feast. But some go hungrier than others, and their ranks are increasing. Some 70 million people in the United States alone are thought to suffer from insomnia or another pathology of sleep.

Sleep is universal, but there is decent evidence that we are doing it wrong. That we need eight hours of sleep a night to function is a myth; that we need our shut-eye in one continued bout is unlikely. Before artificial lights, people went to bed earlier. And it was once more common to have two night-time sleeps, separated by a productive period of wakefulness.

Scientists cannot say for sure how much sleep we need, or when we should take it. As chronobiologist Till Roenneberg points out in a Comment on page 427, part of the reason is that most studies of sleep are done in laboratories. He proposes a radical solution: a US$30-million global human sleep project that would start with online logs of the sleep habits of millions of volunteers and finish with DNA tests to work out where those habits come from.

“The practice of going to sleep and waking at unnatural times could be the most prevalent high-risk behaviour in modern society,” says Roenneberg. Many workers at present, he says, could suffer from a form of social jet lag, forced to shuffle sleep patterns between the conflicting time zones of working and work-free days. That could cause poor health — both physical and mental. The solution would be a profound change: restructure work and school schedules to better suit the biological clocks of the majority of the population, once we work out what they are.

The modern world fragments time. We work on call and watch 24-hour news. Television is on-demand and breakfast usually available all day. We sleep when we can, if we can. Sleep has become another demand on us, and one that we like to allot to a specific window of our daily diary. That is a difficult habit to break for scientists as much as anybody, given their often long hours and frequent travel.

People in many countries get as much as two hours less sleep a night than their ancestors did a century or so ago. That must have a consequence. Lack of sleep may not make our hungry lives longer, it just feels that way.