Read an Excerpt from The CIRCADIAN CODE


We Are All Shift Workers

If you are a card-carrying shift worker who wakes up in the middle of the night to go to work, returns from work late at night, or stays awake all night, you know how it feels to be living against a primitive, primordial drive to sleep at night and stay awake during the day. But even if you’re not, I’m sure you can remember a time when you were fighting against your internal clock. The truth is, we are all shift workers. There are times in life when we go through chronic sleep disruption, and for many, those habits linger. If you pull an all-nighter at school or work, stay up late studying for a test, have a bad night’s sleep, travel across several time zones, stay awake late into the night to tend to a sick relative, or wake up a few times to feed and change a baby, then you too are a shift worker. A full-time job with long commutes combined with a regular home routine is like working two shifts and going to bed past midnight. Even one late night of partying can be just as disruptive as traveling from one time zone to another: That’s why we call it social jet lag.

The statement that “we are all shift workers” isn’t just an idea. Data points to this fact. For example, Professor Till Roenneberg, a researcher in Munich, surveyed more than 50,000 people in Europe and the United States and found that the majority of people either go to bed after midnight or wake up early with insufficient sleep.1,2 Similarly, people also follow different bedtime schedules on weekdays and weekends. At the 2017 World Sleep Congress, Roenneberg presented his data showing that roughly 87 percent of adults have social jet lag and go to bed at least 2 hours later on the weekend.

About 6 years ago, my lab started monitoring the activity and sleep patterns of close to 200 college students, and we found the same pattern that Roenneberg has reported. So far, there’s been only one person in the whole group who actually went to bed every day at the same time, within half an hour, including on weekends. There has been only one other student who went to bed before midnight for at least two days in a week.

We also monitor pregnant women and working moms with babies, and their patterns are also very erratic. In fact, their patterns are most similar to firefighters, who expect to be awoken a few times every night. For many women, the hardest part of motherhood is working against your clock to stay awake at night and trying to catch up on sleep at odd hours of the day. The only time new moms actually got good sleep, not surprisingly, was when they had some help beyond their spouse/partner, like in-laws or parents who could share some of the work at night.

Working mothers have the roughest time syncing their lives to a daily rhythm because their day is affected by everyone else in the home. Typically, working mothers wake up very early to get breakfast ready for the family, prepare the kids, pack the lunch bags and backpacks, get the kids to school or day care, and then get themselves to work. After dinner, they oversee homework, exercise, or work at home late into the night. As the week goes on, their circadian disruption becomes more severe. For instance, when my daughter was an infant, by Friday my wife would literally fall ill, and it would take her all weekend to recover.

No matter what the cause, we all know how it feels the day after a particularly rough night. You feel sleepy, yet you cannot sleep. Your stomach may feel upset, your muscles are weak, your mind is foggy, and you are certainly not in the mood to hit the gym. It’s as if your body and mind are confused–half of your brain may be telling you that it is time to catch up on lost sleep, but the other half is insisting that it’s daytime and you should not sleep. You may resolve to push on and reach for a strong cup of coffee or energy drink to stamp out the urge to sleep or try to get back into your regular routine as quickly as possible.

A brain on shift work cannot make rational decisions. According to a recent article in Popular Science magazine,3 a single night shift has cognitive effects that can last a week. These lapses in memory or attention can also make us vulnerable to bad habits. A few days of reduced sleep can change our appetite, both for the kinds of foods we crave and how much we want to eat when we stay awake at night. Often, we are prone to eat more calorie-dense junk food late at night when our stomach is meant to rest and repair.

Living in the shift-work zone can also cause difficulty in getting to sleep. Some turn to alcohol or sleeping pills, both of which can trigger depression. But more important, they are addictive remedies that create bad habits that continue even when our lifestyle does not demand us to be awake at night.

And if it weren’t bad enough that a shift-work lifestyle affects the way we feel the next day, our family members are in essence secondhand shift workers, as we may inadvertently disrupt their sleep as they wake up early or stay awake late to match our crazy schedules and keep us company. The effects on their health are equally troubling. For instance, in a 2013 analysis of published papers on the topic, researchers found that children of shift workers not only had more cognitive and behavioral problems as compared to children raised by non–shift workers, they also had a higher incidence of obesity.4

While a day or two of staying awake late into the night, or a couple of days after traveling through a few time zones, may be uncomfortable, repeatedly disrupting your circadian clock can have adverse health consequences, as every system in your body starts to malfunction. It makes the immune system so weak that germs and bugs that don’t usually cause any trouble can upset your stomach or even cause flulike symptoms. It has been well documented that shift workers experience more health problems than non–shift workers, particularly gastrointestinal diseases, obesity, diabetes, and cardiovascular diseases.5,6,7,8,9,10,11,12,13,14,15,16 Surprisingly, the number one cause of death and work disability for active-duty firefighters is not fire or accident–it is heart disease, which is now thought to be linked to a disruption of the circadian rhythm.17, 18 In many studies, shift work increases the risk for certain types of cancer to such an extent that, in 2007, the World Health Organization’s International Agency for Research on Cancer classified shift work as a potential carcinogen.19

If we are all shift workers, then we will all suffer. This is why we have to understand how our circadian clock works, and how to optimize our lifestyle to nurture the natural rhythm of the body.

Which Kind of Shift Worker Are You?

A person who stays awake for more than 3 hours between 10:00 p.m. and 5:00 a.m. for more than 50 days in a year fits the official European definition of a shift worker. Yet I believe we are all shift workers simply due to the way we live our lives. Which kind of shift work do you experience?

Traditional shift work: Roughly 20 to 25 percent of the nonmilitary workforce in any developing or developed country is involved in shift work. This includes emergency responders (firefighters, emergency dispatchers); police; workers in health services (nurses, doctors), manufacturing, construction, utility services, air transportation (pilots, flight attendants, ground staff), ground transportation, and food services; custodial staff; and call center customer support workers.

Shift-work-like lifestyle: This includes high school and college students, musicians, performing artists, new mothers, in-home caregivers, and spouses of shift workers.

Jobs in the gig economy: This includes part-time drivers for ride-share services and food delivery services, flexible workers, and freelancers.

Jet lag: This occurs when you travel across two or more time zones within a day. Nearly 8 million air travelers take to the air each day,20 and half of them travel over at least two time zones.

Social jet lag: This occurs when someone sleeps late and wakes up at least 2 hours later on the weekends. More than 50 percent of the population in modern society experiences social jet lag.

Digital jet lag: This happens when you chat with friends or colleagues that are several time zones away over social networks or digital devices and as a result have to stay awake for more than 3 hours between 10:00 p.m. and 5:00 a.m.

Seasonal circadian disruption: Millions of people living in extreme north and south latitudes (residents of northern Canada, Sweden, Norway, and southern Chile, for example) experience less than 8 hours of daylight during winter and more than 16 hours of daylight in summer. These extreme exposures disrupt their circadian rhythm.


Circadian Rhythms Are Real

We used to believe that our day-night cycles were only guided by the external world: The light in the morning would wake us up, and seeing the moon was our cue to go to sleep. Many scientists discounted the entire field of circadian biology, even until the mid-1970s. While it was known as far back as 1700 that there was an internal clock in plants, the idea that animals and humans were internally driven rather than externally motivated was hard to prove. The common wisdom was that humans, a more evolved species, must be driven by outside or environmental factors beyond the sun and moon.

The plant experiments were easy enough: A plant placed in a dark basement will still move its leaves up and down in a particular rhythm each day.21 Many plants move their leaves up during the day to capture more energy from sunlight. At night, their leaves can drop, because it would be a waste of energy to keep the leaves raised. Similarly, many flowers only bloom during the day, when pollinating bees and birds are flying around, yet some, like the jasmine tree near my grandparents’ home, bloom at night: These plants depend on wind, not other animals, for pollination.

The next set of studies was exponentially more difficult, and scientists started with insects, birds, and then animals. They researched the timing of larvae turning into fruit flies, which is circadian because it only happens in the morning, when there is less wind and more humidity. They studied the migration patterns of birds and the waking patterns of other animals. Laboratory mice were also studied under a controlled environment.22 When they were put under constant darkness without any outside timing cues, they also woke up and went to sleep with clockwork precision, every 23 hours 45 minutes. Similarly, the circadian clocks of many plants and fungi are close to but not exactly 24 hours.

It was almost impossible to investigate if humans had these same internal clocks because there was no easy way to remove all of the external timing cues of every connection to the outside world. However, in the 1950s, researchers had an idea: They created a simple telephone that could connect a volunteer to only one other person. The volunteer went deep into a cave far in the Andes Mountains. All he brought with him was enough food, candles, and reading materials to keep him occupied for weeks. Each time he felt sleepy enough to go to bed, he would call his partner on the other side of the phone, who would record the time. He would make the same call when he awoke. The study showed that his sleep-wake cycle continued with clockwork precision for several weeks in the cave. However, the volunteer went to bed a little later every day, implying his clock was slightly longer than 24 hours. In fact, he was going to bed and waking up within a cycle that covered exactly 24 hours 15 minutes. His cycle was so predictable that it could only be guided by an internal clock.23


1 D. Fischer et al., “Chronotypes in the US–Influence of Age and Sex,” PLoS ONE 12 (2017): e0178782.

2 T. Roenneberg et al., “Epidemiology of the Human Circadian Clock,” Sleep Medicine Reviews 11, no. 6 (2007): 429-38.

3 L. Kaufman, “Your Schedule Could Be Killing You,” Popular Science, September/October 2017, https://www.popsci.com/your-schedule-could-be-killing-you.

4 J. Li et al., “Parents’ Nonstandard Work Schedules and Child Well-Being: A Critical Review of the Literature,” Journal of Primary Prevention 35, no. 1 (2014): 53-73.

5 D. L. Brown et al., “Rotating Night Shift Work and the Risk of Ischemic Stroke,” American Journal of Epidemiology 169, no. 11 (2009): 1370-77.

6 M. Conlon, N. Lightfoot, and N. Kreiger, “Rotating Shift Work and Risk of Prostate Cancer,” Epidemiology 18, no. 1 (2007): 182-83.

7 S. Davis, D. K. Mirick, and R. G. Stevens, “Night Shift Work, Light at Night, and Risk of Breast Cancer,” Journal of the National Cancer Institute 93, no. 20 (2001): 1557-62.

8 C. Hublin et al., “Shift-Work and Cardiovascular Disease: A Population-Based 22-Year Follow-Up Study,” European Journal of Epidemiology 25, no. 5 (2010): 315-23.

9 B. Karlsson, A. Knutsson, and B. Lindahl, “Is There an Association between Shift Work and Having a Metabolic Syndrome? Results from a Population Based Study of 27,485 people,” Occupational & Environmental Medicine 58, no. 11 (2001): 747-52.

10 T. A. Lahti et al., “Night-Time Work Predisposes to Non-Hodgkin Lymphoma,” International Journal of Cancer 123, no. 9 (2008): 2148-51.

11 S.  P.  Megdal et al., “Night Work and Breast Cancer Risk: A Systematic Review and Meta-Analysis,” European Journal of Cancer 41, no. 13 (2005): 2023-32.

12 F. A. Scheer et al., “Adverse Metabolic and Cardiovascular Consequences of Circadian Misalignment,” Proceedings of the National Academy of Sciences of the United States of America 106, no. 11 (2009), 4453-58.

13 E. S. Schernhammer et al., “Night-Shift Work and Risk of Colorectal Cancer in the Nurses’ Health Study,” Journal of the National Cancer Institute 95, no. 11 (2003): 825-28.

14 E. S. Schernhammer et al., “Rotating Night Shifts and Risk of Breast Cancer in Women Participating in the Nurses’ Health Study,” Journal of the National Cancer Institute 93, no. 20 (2001): 1563-68.

15 S. Sookoian et al., “Effects of Rotating Shift Work on Biomarkers of Metabolic Syndrome and Inflammation,” Journal of Internal Medicine 261, no. 3 (2007): 285-92.

16 A. N. Viswanathan, S. E. Hankinson, and E. S. Schernhammer, “Night Shift Work and the Risk of Endometrial Cancer,” Cancer Research 67 no. 21 (2007): 10618-22.

17 E. S. Soteriades et al., “Obesity and Cardiovascular Disease Risk Factors in Firefighters: A Prospective Cohort Study,” Obesity Research 13, no. 10 (2005): 1756-63.

18 E. S. Soteriades et al., “Cardiovascular Disease in US Firefighters: A Systematic Review,” Cardiology in Review 19, no. 4 (2011): 202-15.

19 K. Straif et al., “Carcinogenicity of Shift-Work, Painting, and Fire-Fighting,” Lancet Oncology 8, no. 12 (2007): 1065-66.

20 International Air Transport Association, “New Year’s Day 2014 Marks 100 Years of Commercial Aviation,” press release, http://www.iata.org/pressroom/pr/Pages/2013-12-30-01.aspx.

21 J.-J. de Mairan, “Observation Botanique,” Histoire de l’Academie Royale des Sciences (1729): 35-36.

22 J. Aschoff, “Exogenous and endogenous components in circadian rhythms.” Cold Spring Harbor Symposia on Quantitative Biology 25 (1960): 11-28.

23 J. Aschoff and R. Wever, “Spontanperiodik des Menschen bei Ausschluß aller Zeitgeber,” Naturwissenschaften 49, no. 15 (1962): 337-42.


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