Trying to sleep during the wrong phase of the circadian rhythm - either too early, too late, or at completely the wrong time of the day - means that a shift worker’s sleep period may coincide with the “wake maintenance zone” of their daily circadian cycle, leading to a long “sleep latency” (the time it takes to get to sleep) and reduced overall sleep time. But is also alters the type and quality of sleep, as well as affecting hormone levels and metabolism.
In real world conditions, it usually takes about a day for our circadian rhythms to adapt just one hour (see the section on Jet Lag), so that changing from an 8am day shift to an 8pm night shift, for example, would normally require about 12 days. In practice, of course, no time is allowed for such adjustments. The problem is exacerbated by transition periods (e.g. a night shift for five consecutive nights, followed by two days and two nights off, during which time the person may revert to a typical nighttime-sleep/daytime-awake schedule in order to spend time with family and friends). Rotating shifts (e.g. one week from 7am to 3pm, the next week from 3pm to 11pm, the third week from 11pm to 7am) are even more disruptive than a steady night shift, because shifts change so rapidly that the worker’s circadian rhythm never has time to adapt from one shift to another.
Shift worker disorder is sometimes recognized as a formal sleep disorder, despite the fact that it does not arise from internal medical conditions but rather from outside social pressures and, at least to some extent, personal lifestyle choices. Even long-time night shift workers typically get about two hours less sleep per night than the average person, partly due to the circadian rhythm effect, but partly due to practicalities of social routines, additional noise during the day, etc. An estimated 10% of night and rotational shift workers develop a full-blown disorder, with some studies suggesting that older individuals may be more at risk than younger people.
On a day-by-day basis, shift workers typically suffer from fatigue, poor performance, poor memory, indigestion, gastrointestinal problems, and other sleep deprivation symptoms. But they are also at increased risk of ongoing health problems like chronic fatigue, cardiovascular disorders, depression, diabetes, ulcers, and some forms of cancer.
The sleep debt accumulated as a result of shift work makes workers much more vulnerable to accidents and injuries. It is no coincidence that many high profile industrial and transportation accidents (e.g. Exxon Valdez, Chernobyl, Bhopal, Three Mile Island, etc) occurred at night, and the risk of accidents in general is estimated at about 30% higher on night shifts than on day shifts, with the risk increasing with the duration of the shift and the number of consecutive night shifts worked. A sobering 2004 study reported that, among first-year medical residents (trainee doctors), who are often called upon to work 24-hour or even 30-hour shifts, about one in five admitted to making a fatigue-related mistake that resulted in injury to a patient, and one in twenty to a fatigue-related mistake resulting in a death.
A few simple but effective measures can be taken - although they seldom are - to mitigate the effects of shift work. Increasing the intensity of light in the workplace, and shifting the wavelength of light toward the blue end of the spectrum (to make it more similar to natural daylight) may help workers to adapt to working at night, and may even enhance their ability to sleep during the daytime. It is possible to trick the body’s biological clock to some extent by wearing dark glasses when returning home in the morning, and generally limiting after-work, pre-sleep exposure to light. Strategic use of caffeine can also help shift workers stay awake, especially if they consume regular, small doses of caffeine (rather than fewer, large doses), and taper off caffeine consumption halfway through their shift so that their caffeine levels have time to dissipate before sleep. Breaks for naps of 20-30 minutes have also been shown to improve night shift performance.