Shift work sleep disorder

Shift work sleep disorder (SWSD) is a circadian rhythm sleep disorder characterized by insomnia and excessive sleepiness affecting people whose work hours overlap with the typical sleep period. There are numerous shift work schedules, and they may be permanent, intermittent, or rotating; consequently, the manifestations of SWSD are quite variable.

Diagnosis

The primary symptoms of shift work sleep disorder are insomnia and excessive sleepiness associated with working (and sleeping) at non-standard times. Shift work sleep disorder is also associated with falling asleep at work. Total daily sleep time is usually shortened and sleep quality is less in those who work night shifts compared to those who work day shifts.^[1] Sleepiness is manifested as a desire to nap, unintended dozing, impaired mental acuity, irritability, reduced performance, and accident proneness. Shift work is often combined with extended hours of duty, so fatigue can be a compounding factor.^[2] The symptoms coincide with the duration of shift work and usually remit with the adoption of a conventional sleep-wake schedule.^[2] The boundary between a "normal response" to the rigors of shift work and a diagnosable disorder is not sharp.

Causes and comorbidities

Insomnia and wake-time sleepiness are related to misalignment between the timing of the non-standard wake–sleep schedule and the endogenous circadian propensity for sleep and wake. In addition to circadian misalignment, attempted sleep at unusual times can be interrupted by noise, social obligations, and other factors. Finally, there is an inevitable degree of sleep deprivation associated with sudden transitions in sleep schedule.^[1]

Health risks

There have been many studies suggesting health risks associated with shift work. For example, a 2007 study led by the IARC (International Agency for Research on Cancer) showed that shiftwork has been associated with cancer.^[1][3] Other studies have reported that night workers have an increased incidence of heart disease, digestive disorders and menstrual irregularities.^[4] Because a formal diagnosis of SWSD was not typically made in these studies, it remains unclear whether the reported risks apply to the subset of shiftworkers who qualify for a diagnosis of SWSD or apply to all shiftworkers.

Suprachiasmatic nucleus (SCN)

Brain arousal is stimulated by the circadian system during the day and sleep is usually stimulated at night.^[5] The rhythms are maintained in the suprachiasmatic nucleus (SCN), located in the anterior hypothalamus in the brain, and synchronized with the day/night cycle.^[5] Gene-transcription feedback loops in individual SCN cells form the molecular basis of biological timekeeping.^[5] Circadian phase shifts are dependent on the schedule of light exposure, the intensity, and previous exposure to light.^[6] Variations in exposure can advance or delay these rhythms. For example, the rhythms can be delayed due to light exposure at night.^[6]

Photoreceptors located in the retina of the eye send information about environmental light through the retinohypothalamic tract to the SCN.^[5][6] The SCN regulates the pineal gland, which secretes the hormone melatonin.^[6] Typically, the secretion of melatonin begins two hours before bedtime and ends two hours prior to waking up.^[6] A decline in neuronal firing in the SCN is caused by the binding of melatonin to the MT1 and MT2 melatonin receptors. It is believed that the reduction in firing in the SCN stimulates sleep.^[5] While day-active individuals produce melatonin at night, night shift workers' production of melatonin is suppressed at night due to light exposure.^[6]

Treatment

Prescribed sleep/wake scheduling

Experts agree that there is no such thing as an "ideal" night work schedule, but some schedules may be better than others. For example, rotating shifts every two weeks in a forward (delaying) direction was found to be easier than rotation in a backward (advancing) direction.^[7] Gradual delays (“nudging” the circadian system about an hour per day) has been shown in a laboratory setting to maintain synchrony between sleep and the endogenous circadian rhythms,^[8] but this schedule is impractical for most real world settings. Some experts have advocated short runs (1 to 2 days) of night work with time for recovery; however, in the traditional heavy industries, longer (5 to 7 day) runs remain the rule. In the end, scheduling decisions usually involve maximizing leisure time, fairness in labor relations, etc. rather than chronobiological considerations. Shift workers can benefit from adhering to sleep hygiene practices related to sleep/wake scheduling. Symptoms typically only fully resolve once a normal sleep schedule is resumed.^[2]

Many night workers take naps during their breaks, and in some industries, planned napping at work (with facilities provided) is beginning to be accepted. A nap before starting a night shift is a logical prophylactic measure. However, naps that are too long (over 20–30 minutes) may generate sleep inertia, a groggy feeling after awakening that can impair performance. Therefore, brief naps (10 to 30 minutes) are preferred to longer naps (over 30 minutes). Also, long naps may also interfere with the main sleep bout.

In the transportation industry, safety is a major concern, and mandated hours of service rules attempt to enforce rest times.

Bright light treatment

The light-dark cycle is the most important environmental time cue for entraining circadian rhythms of most species, including humans, and bright artificial light exposure has been developed as a method to improve circadian adaptation in night workers. The timing of bright light exposure is critical for its phase shifting effects. To maximize a delay of the body clock, bright light exposure should occur in the evening or first part of the night, and bright light should be avoided in the morning. Wearing dark goggles^[9] (avoiding bright light) or blue-blocking goggles during the morning commute home from work can improve circadian adaptation. For workers who want to use bright light therapy, appropriate fixtures of the type used to treat winter depression are readily available^[10] but patients need to be educated regarding their appropriate use, especially the issue of timing. Bright light treatment is not recommended for patients with light sensitivity or ocular disease.

Melatonin treatment

Melatonin is a hormone secreted by the pineal gland in darkness, normally at night. Its production is suppressed by light exposure,^[11] principally blue light around 460 to 480 nm. Light restriction, or dark therapy, in the hours before bedtime allows its production. Dark therapy does not require total darkness. Amber or orange colored goggles eliminate blue light to the eyes while allowing vision.

Melatonin is also available as an oral supplement. In the US and Canada, the hormone melatonin is not classified as a drug; it is sold as a dietary supplement. In other countries it requires a prescription or is unavailable. Although it is not licensed by the FDA as a treatment for any disorder, there have been no serious side effects or complications reported to date.

Melatonin has been shown to accelerate the adaptation of the circadian system to a nighttime work schedule.^[12] Melatonin may benefit daytime sleep in night workers by an additional direct sleep promoting mechanism. Melatonin treatment may increase sleep length during both daytime and nighttime sleep in night shift workers.^[1]

Medications that promote alertness

Caffeine is the most widely used alerting drug in the world and has been shown to improve alertness in simulated night work.^[13] Caffeine and naps before a night shift reduces sleepiness during the shift.^[1] Modafinil and armodafinil are non-amphetamine alerting drugs originally developed for the treatment of narcolepsy that have been approved by the FDA (the US Food and Drug Administration) for excessive sleepiness associated with SWSD.^[14]

Medications that promote daytime sleep

Obtaining enough sleep during the day is a major problem for many night workers. Hypnotics given in the morning can lengthen daytime sleep; however, some studies have shown that nighttime sleepiness may be unaffected.^[15] Zopiclone has been shown to be ineffective in increasing sleep in shift workers.^[1]

See also

• Shift work
• Jet lag
• Human factors
• Human reliability

References

External links

• Shift Work Sleep Disorder (uni-marburg.de)
• (IARC/usnews)