The Clock Inside

1At four o’clock in the morning, New York is quiet, but it is not asleep. Nurses are checking on patients. Bus operators are driving the overnight routes. Bakers are shaping bread, and security guards are walking their rounds. In the United States, about 15 percent of wage and salary workers usually work a schedule other than a regular daytime one. Many of them know a feeling that comes in the hours before dawn: a heavy wave of sleepiness that has little to do with how much they slept the day before. That wave comes from a clock inside the body.

2Scientists call the body’s daily cycle a circadian rhythm. The word comes from Latin: circa means “about,” and dies means “day.” Over about 24 hours, the body goes through a regular pattern of changes. Body temperature rises and falls. Hormones, the chemicals that carry messages through the blood, go up and down. So do hunger and alertness. The master clock that keeps this schedule is a small group of nerve cells deep in the brain, called the suprachiasmatic nucleus, or SCN.

3The clock is not perfect on its own. In experiments where people lived for weeks without clocks or daylight, their body clocks ran a little longer than 24 hours, so they drifted later and later. What keeps the clock on time in ordinary life is light. Light enters the eyes, and the eyes send a signal to the SCN. Morning light resets the clock every day. In the evening, as the light fades, the SCN signals the body to make more melatonin, a hormone that makes us sleepy.

4How does a group of cells keep time? In the 1980s, three American scientists, Jeffrey Hall, Michael Rosbash, and Michael Young, studied the question in fruit flies. They worked with a gene called period, which other scientists had found in fruit flies in 1971. The gene tells a cell to make a certain protein. The protein builds up in the cell during the night and breaks down during the day, and when enough of it has built up, it shuts off its own production. That rise and fall takes about 24 hours. In 2017, the three scientists won the Nobel Prize for this work. Later research showed that nearly every cell in the human body has a clock of its own. The master clock in the brain keeps them all on the same schedule.

5Night work goes against this clock. A night worker is awake when the clock says to sleep, and tries to sleep when both the clock and the daylight say to wake up. Sleep during the day is usually shorter and lighter than sleep at night. There is sunlight at the edges of the curtains, traffic outside, and a family on a daytime schedule. Even after years of night work, most people’s clocks never fully switch. On days off, they go back to sleeping at night, and the struggle starts over. The hardest time is usually the early morning, before dawn, when the clock is at its lowest point. That is also when many night workers drive home.

6Over months and years, the struggle can take a toll. According to the National Institutes of Health, a body clock that is thrown off for a long time can raise the risk of obesity, diabetes, depression and other mood disorders, heart and blood pressure problems, and cancer. In 2019, a panel of experts working for the World Health Organization looked at the research on cancer and concluded that night shift work probably causes cancer in humans. Probably was a careful choice of word. It meant that the evidence was strong enough to worry about, but not strong enough to be sure.

7Night workers cannot change their jobs just by wishing, but there are things that help. The National Institute for Occupational Safety and Health, a federal agency that studies workplace health, recommends bright light during the shift, which tells the body it is daytime. On the way home, it recommends wraparound sunglasses that block blue light, kept on until the worker is in a dark bedroom. The bedroom should be dark and quiet. And a worker who is very sleepy should not drive: it is safer to ride with someone else, take a taxi, or take public transportation.

8Not all of this is up to the worker. Researchers who study shift work point out that the schedule matters too. When shifts rotate, moving forward, from days to evenings to nights, is easier on the body than moving backward, because the body clock finds it easier to shift later than earlier. Fewer night shifts in a row, and enough time off between shifts to sleep, also help. Those decisions are made by employers, and in many workplaces they are worked out with a union.

9The city needs people working at night. Someone has to care for the sick at three in the morning, drive the buses, and bake the bread that will be sold at seven. The human body was not built for that schedule. The question that science raises is how to make the work less harmful, for the people who keep the city running while everyone else sleeps.