- Traveling across time zones can disrupt regular patterns of sleep and wakefulness.
- Because the internal circadian alerting system is at its strongest during the daytime, night shift workers often find themselves struggling to sleep "on the wrong side of the clock."
- Good sleep habits (in particular), a regular schedule, and simple workplace measures can help shift workers get the sleep they need.
If It Feels Like Midnight, This Must Be Paris
If you've ever traveled by plane across several times zones, chances are you've experienced "jet lag." Symptoms of jet lag may include excessive daytime sleepiness, nighttime insomnia, headache, loss of appetite, gastrointestinal problems, and irritability or mild depression.
Jet lag is a result of the mismatch between the external environment and our internal biological clock. This clock is the pacemaker system that controls many different bodily functions during a 24-hour period and regulates when we sleep and when we wake up. Because plane travel is quick, someone who crosses several time zones will experience a temporary desynchronization between the new time zone schedule and his or her internal clock. For example, if you take a six-hour flight from Boston to Paris and your plane leaves at 6:00 p.m., it will touch down at 6:00 a.m. Paris time. However, to your body it will feel like the middle of the night—because back in Boston it actually is midnight.
Shifting to a new time zone in this way may result in sleep disturbances. This is due to circadian misalignment: your body's internal clock is out of synch with the actual time in the new time zone. Your body feels that it is time to go to sleep when others are having breakfast, and you feel wide awake when everyone around you is fast asleep.
Fortunately, jet lag is usually temporary because our internal biological clock adapts in response to external cues in the new environment. That adaptation—in which the internal clock readjusts itself a little bit every day until it is in the normal alignment with the external environment—is called entrainment. The environmental cues that nudge the entrainment process along include exposure to light in the first few days following travel, being active, and eating meals and sleeping at appropriate times in the new time zone. It is usually advisable to avoid long daytime naps in the new time zone in order to build up enough of a drive to sleep to promote nighttime sleep.
Dr. Dennis Dean describes how jet lag affects the body and suggests ways to minimize the negative effects.
Of all the environmental cues, light is the most powerful synchronizer of our internal biological clock. It is the cue for the body to know when it should be awake and active. It is difficult to predict what new schedule an individual should adopt in order to minimize the effects of jet lag. It depends on many factors: the time of day of travel, the speed, distance, and direction of travel, the person’s internal clock time, the person’s habitual sleep/wake schedule, and the light/dark schedule.
The general rule of thumb for eastward flights—from North America to Europe, for example—is that travelers should avoid bright light when it is late evening and night according to their internal clock (or their usual nighttime). And conversely, travelers should seek light when it is their usual early morning (generally starting at about 5 a.m. internal time). The objective is to signal to the body that it is an early sunrise—which will result in shifting the internal biological clock earlier toward the correct alignment for the new time zone.
Unfortunately, if a person experiences light much earlier than 5 a.m. internal time, then the body interprets this as a late sunset rather than an early sunrise. This will result in shifting the internal biological clock in the wrong (or later) direction. This undesired effect would exaggerate the misalignment between the internal clock and the new time zone, and result in more pronounced jet lag. In other words, let's say you take that overnight flight from Boston to Paris that arrives at 6 a.m. local time. When you arrive, your body feels like it is midnight. To adjust most quickly to the new time zone, you should avoid bright sunlight ideally for at least five hours and then seek light after 11 a.m. Generally speaking, the internal clock resets at a rate of about one hour (or time zone) per day. Some body systems and organs may take even longer to adjust to the change. Click on the interactive activity below to learn more about how shifts in your internal biological clock can produce issues with your ability to stay awake or fall asleep at appropriate times.
The Challenges of Shift Work
Unlike travelers who experience jet lag (a temporary disruption of the body's internal biological clock due to travel across time zones), shift workers—employees who work regular shifts at night or alternate among different shifts—experience longer-term disruptions that carry social as well as physiological implications.
People who work an ongoing night shift (for example, from 11 p.m. to 7 a.m.) must adapt to sleeping during daylight hours and being awake and alert during the night. Although it is possible to adjust one's schedule to sleep during the daytime, the ability to sleep solidly is a big challenge for many people because their internal clock is usually set to be awake during the day. Similarly, staying awake and alert at night is a challenge for many people because their circadian alerting system is at its lowest point, and from their body’s perspective, the urge to succumb to sleep is quite strong.
Dr. Charles Czeisler describes some of the sleep problems that are typical to shift workers.
In addition, the practical, day-to-day responsibilities of family life, friends, and social obligations make it difficult for a night shift worker to completely transition to a daytime sleep schedule. For example, a person may work the night shift for five consecutive nights, followed by two days and two nights off. During this "weekend," the person may revert to a typical nighttime-sleep/daytime-awake schedule in order to spend time with family and friends. This causes an individual's internal clock to shift again, thus requiring another adjustment when the night shift workweek begins. Without a constant sleep/wake schedule during the entire week, the body’s internal biological rhythms may always remain out of sync.
Dr. Dennis Dean describes how social factors and family obligations make sleeping during the day difficult for those working at night.
Similar problems may occur for those shift workers who cycle among three different shifts (for example, one week from 7 a.m. to 3 p.m., the next week from 3 p.m. to 11 p.m., the third week from 11 p.m. to 7 a.m.). Changing from one shift schedule to another each week can be disorienting in a similar way as jet lag. In this situation, the body's sleep-wake schedule is continually desynchronized—that is, the sleep-wake routines are constantly shifting and the body cannot adjust quickly enough to the differing external cues each week. Moreover, the body cannot adequately rest and rebuild when the circadian rhythms are so frequently disrupted.
Shift Work Disorder
Some people who work exclusively at night or who work rotating shifts can experience shift work disorder, a chronic condition that is directly related to their work schedule. The two primary symptoms of shift work disorder are insomnia when a person is trying to sleep, and excessive sleepiness when a person needs to be awake and alert.
Insomnia is very common among individuals who sleep during the day. Since the circadian alerting system is typically at its strongest during the daytime, night shift workers find themselves struggling to sleep "on the wrong side of the clock." Just as undesirable is the fact that individuals who work at night will often experience bouts of extreme drowsiness while they are working. Indeed, about half of all night shift workers report falling asleep at work. When you consider that shift workers include large numbers of long-haul truck drivers, pilots, train operators, medical providers, and individuals responsible for maintaining our 24-hour infrastructure, shift work disorder is a serious public health issue.
Chronic sleep deprivation is one outcome of shift work disorder. Night shift workers, on average, sleep about one and one-half fewer hours per night than daytime workers. Lack of sleep clearly affects alertness, judgment, and performance. The increased rate of work-related accidents at night and motor vehicle accidents following a night shift are evidence that long-term shift work can be hazardous to health and well-being. Recent research suggests that chronic sleep loss may lead to an increased risk for obesity, diabetes, and cardiovascular disease.
For more about the consequences of insufficient sleep, see Sleep and Disease Risk and Sleep, Performance, and Public Safety.
Measures to Help Shift Workers
In addition to shift workers' getting adequate sleep and maintaining a consistent sleep schedule, there are workplace measures that can be implemented to help them adapt to their schedules and maintain alertness during work. Studies have demonstrated that enhancing lighting in the workplace—increasing the intensity of light and shifting the wavelength of light toward the blue end of the spectrum (to simulate daylight)—makes it easier for workers to adapt to working at night. These changes may even enhance our ability to sleep during the daytime. Strategic use of caffeine can also help shift workers stay awake. It is better if workers consume regular, small doses of caffeine than fewer, large doses, and taper off halfway through their shift if they plan to go home and sleep immediately after work. Breaks for 20- to 30-minute naps have been shown to improve performance.
Dr. Charles Czeisler provides tips on how to better work at nights.
If shifts must rotate, it is easier for workers to adjust to a schedule that rotates from day shift to evening shift to night shift than it is to adjust to a reverse rotation. If sleep or job alertness problems are persistent and severe, shift workers should consider visiting a sleep specialist for an evaluation of a possible underlying sleep disorder that may be complicating the effects of shift work.
It is critical that shift workers protect their sleep time and pay close attention to their sleep environment. Creating a dark, comfortable, and quiet sleep environment means eliminating anything that could disrupt sleep: turning off the phone during the day, disconnecting the doorbell, and making arrangements for someone else to pick up the children from school. More and more evidence is emerging that getting adequate sleep is essential for our safety, both on the job and off, as well as our overall mood, quality of life, productivity, and health.
© 2023 by the President and Fellows of Harvard College