Jet lag is caused by undergoing a rapid time-zone transition (Waterhouse, 2005). When an individual travels from one time zone to another in a short time, his/her internal circadian rhythm and the new time zone will be mismatched which results in the situation of circadian desynchronosis (Benca, 2012 and Campbell, 2002). The severity of the problem depends on the number of time zones crossed (Bayer, 2001). The short-term effects of jet lag include the inability to initiate and consolidate sleep, a reduction in overall sleep duration, and a reduction of REM sleep and increase of NREM sleep (Costa, 1999 and Mallis, 2010). These effects usually last for few days because the circadian system resynchronizes slowly to the zeitgebers of the destination time zone (Campbell, 2002 and Graeber, 1988). Since a cycle of normal circadian rhythm is a little bit more than 24 hours, eastward travel requires a longer period of adjustment than westward travel across equivalent time zones because of the circadian rhythm tends to cope with the phase delay (Benca, 2012 and Costa, 1999).
Working on Shift
Shift workers may be required to work during their normal sleep hours and sleep during normal waking hours (Benca, 2012). Due to the internal and external desynchronosis, the circadian cycle has to be adjusted to the new schedule after moving to a new shift (Caldwell, 2006 and Campbell, 2002). These adjustments include changing the normal sleep-wake pattern and adjusting the body functions, to satisfy the demands of working when the shift workers should sleep and sleep when they should awake (Costa, 1999). However, before being adapted to the new schedule, the quantity and quality of sleep would both decrease (Costa, 1999). Morning shift workers usually have lesser hours of sleep due to the early wakeup, which also causes a reduction of REM sleep (Costa, 1999). In addition, daytime sleep is often shortened, not only due to the inability to sleep resulting from the desynchronosis, but also because of the unfavorable daytime environmental disturbances, such as ringing telephones and traffic noises (Bayer, 2001 and Benca, 2012).
Psychological stress is commonly resulted from the environmental demands in workplaces, such as downsizing, tight deadlines, long working hours, and heavy workload (Caldwell, 2006 and Ewing, 2003). Such stressful conditions would decrease the quality of sleep as a result of the increased tension and difficulties in initiating or maintaining sleep (Akerstedt, 1987). The sleep patterns would also be disrupted temporarily, in terms of intensified REM sleep, reduced total sleep time, reduced time of deep sleep and increased number of awakenings during sleep, until the stressful situation is soothed (Akerstedt, 1987 and Caldwell, 2006).