Introduction
Shift work is basically any work outside the normal daylight hours and may differ from one work place to another. Given that up to 25% of the American population encounters shift work at some point in their career the subject of shift work has attracted a plethora of research especially with regard to the health implication of shit work[ CITATION Occ10 \l 1033 ]. There is a growing body of knowledge implicating shift work for on-job injuries and catastrophes, disruption of families and societal fabric, pathological conditions of both physiological and psychological nature. Of particular interest to this review are the health implications of shift work thus the purpose of this review is to elucidate some of the specific health problems associated with shift work identifying research evidence of these associations as well as the mechanisms by which shift work causes or aggravates the conditions.
There is a growing body of knowledge indicating that shift work is empirically associated with various diseases as well as aggravating already existing ones[ CITATION AKn03 \l 1033 ]. Shift work and particularly night shift has been positively linked to various forms of cancer and there is a lot of research attempting to demonstrate and explain this association[ CITATION ESS04 \l 1033 ]. Some studies have also demonstrated an association between shift work and reproductive health problems more so in women[ CITATION SLa02 \l 1033 ]. It has also been found that shift work is crudely associated with increase in body weight[ CITATION Ava11 \l 1033 ]. It also been reported that many shift workers complain of gastrointestinal and digestive problems[ CITATION CCC04 \l 1033 ]. Finally for decades several research findings have concluded that there is a definite and strong correlation between shift work and cardiovascular diseases[ CITATION HBo99 \l 1033 ]. It is important therefore to review the mechanisms underlying shift work and disease association.
Mechanisms by which shift work contributes to health problems
At least three mechanisms have been identified in the association between shift work and disease. The first pathway involves the disruption of the circadian rhythm. The circadian rhythm is simply the occurrence of distinct rhythmic changes/patterns in the physiological functions within a 24 hour period as influenced by light and the core body temperature. The disruption of these rhythmic changes results in sleep disturbances, increased disease susceptibility and more importantly desynchronizes the body internal processes. The second pathway has to do with the disturbance of the social aspects of life. Shift workers often have family problems, little social support and stress all of which contribute to health problems and particularly of a psychological nature. Shift work has also been implicated in contributing to unhealthy and destructive habits. Such unhealthy practices as increased smoking, excessive alcohol consumption, irregular meals and poor diet have been linked to disease by various studies. In addition to these three mechanisms there is evidence of changes in biomarkers such as plasminogen, cholesterol, lipids, blood pressure, hormones and cardiac activity which may mediate the pathogenesis of disease[ CITATION AKn03 \l 1033 ].
Of particular interest is the psychological pressure associated with shift work which results in stress. Stress and depression have been linked to various disease processes and the drugs used to control them may have negative side effects. The change of analytes in the bodies of shift workers is of particular interest to diagnosticians as it may lead to wrong diagnosis thus unnecessary or wrong intervention strategies[ CITATION HBo99 \l 1033 ]. It is however important to consider other factors when studying health implications of shift work particularly because most shift workers work in conditions with peculiar hazards. Shift works are also vulnerable to many on-job injuries which must be considered in the distribution of tasks. There is also growing evidence that women are particularly susceptible to some of the shift work related health concerns.
Shift work and body weight
A plethora of research suggests that there is a positive correlation between shift work and body weight[ CITATION Ava11 \l 1033 ]. Drongelen et al (2011) systematically reviewed and summarized eight research articles to decipher the effects of shift work on body weight. Their review concluded that there is indeed a crude association between shift work and increase in body weight. The study met the reliability requirements in that only longitudinal studies were reviewed which was vital because there can be occasional and temporary increase in body weights in workers due to other factors thus the need to follow the workers for a long period to establish association. The major weakness of this study was the quality and size of the sample. The researchers rated five out of the eight articles as high-quality and used the results of the five articles that provided weight changes adjusted for other relevant confounders to make their conclusion. Out of the five studies only two established that the difference between shift workers and non shift workers was statistically significant. The reviewers themselves considered the evidence insufficient to make a definitive conclusion and recommended use of more high quality data to clarify the mechanisms involved.
An earlier ten year study identified shift work as a risk factor for excessive weight[ CITATION Eff07 \l 1033 ]. The study involved 1529 male workers that were grouped into four categories; daytime workers, those who changed from shift work to fixed daytime, those who changed from fixe daytime to shift work and those who remained in shift work for the entire study period. Changes in body mass index (BMI), serum cholesterol, blood pressure and glycosylated haemoglobin A1C for the groups over the 10 years period were recorded and compared. The researchers found that after adjusting for the relevant confounders the BMI increase for those changing from day to shift work was significantly higher than for those working daytime fixed schedule and those changing from shift work to daytime fixed work. In addition workers that remained on shift work the entire period had a much higher BMI increase than those working fixed daytime schedules. Given the sample size, the length of time over which the study was conducted and the design of the research the conclusion of the study that shift work has the potential to increase BMI is reliable.
Shift work and cancer
Shift work has been associated with various forms of cancer to the extent that the International Agency for Research on Cancer (IARC) concluded that shit work could probably be carcinogenic to humans[ CITATION IAR07 \l 1033 ]. Evidence linking shift work to cancer is largely particular to breast cancer and very little has been reported on other forms of cancer. Studies indicate that there is an overall risk ratio for breast cancer of 1.5 associated with shift work and that the risk increases with age, length of exposure to light at night and the intensity of the light[ CITATION JHa01 \l 1033 ].
According to one study exposure to light at night is what actually increases the risk to breast cancer because of reduced nocturnal melatonin production[ CITATION Sco01 \l 1033 ]. To validate their hypothesis Davis et al (2010) also studied women sleeping with lights on over a period of 10 years. They concluded that in deed the suppression of melatonin, which in turn increases estrogen secretion, by light increases the risk of breast cancer. Reproductive hormones and particularly estrogen increase the proliferation of hormone sensitive cells in the breast[ CITATION ESS04 \l 1033 ]. The same mechanism has been implicated in negatively impacting the reproductive health of women especially in irregular menstruation[ CITATION SLa02 \l 1033 ]. A recent study has also reported increased colorectal cancer in female nurses working night shifts[ CITATION ESS03 \l 1033 ]. All the studies linking shift work to cancers have focused on female workers and it would therefore be necessary to explore similar studies with male subjects or even mixed gender comparative studies. Shift work has also been shown to adversely affect pregnancy outcomes increasing the risk of premature births, low birth weights and spontaneous abortion[ CITATION AJS00 \l 1033 ].
Shift work and gastrointestinal problems
Gastrointestinal and digestive disorders have been reported more often in shift workers and night workers than in day workers. The most common complains in this category have been heartburn, indigestion, loss of appetite and stomachache. One study established that there were more complains of gastrointestinal problems among nurses on rotational shifts than among their colleagues on regular day shifts. The study involving 160 nurses from Shahid Beheshti Hospital in Iran concluded that GI symptoms were significantly higher among the former group (81.9%) compared to the latter (59.2%). The highly stratified study further established that the proportions of GI problems were higher in female nurses (84.8% and 55.5%) than in male nurses (72.5% and 66.7%). There is also evidence linking shift work to more serious conditions such as peptic and duodenal ulcers[ CITATION AKn03 \l 1033 ]. The irregularities in the work, types and time of meals, fatigue and social stress have been implicated in the etiology of the GI disorders. The loss of appetite in shift workers results in eating of more junk food rather than balanced diets. Another study established that there was a higher prevalence of irritable bowel syndrome (IBS) among nurses working in rotational shifts compared to those working regular day shift. This particular study implicated disturbance of the circadian rhythm for the high prevalence of IBS in nurses working in rotational shifts[ CITATION AJS00 \l 1033 ].
Shift work and cardiovascular disorders
For the last two decades evidence suggesting that shift work significantly increases the risk of cardiovascular diseases (CHD) has been documented. In a recent study review Knutsson (2003) concluded that there is strong evidence that shift work is associated with coronary heart disease. The overall risk for cardiovascular disease among shift worker has been reported to be 40% higher than among workers working fixed daytime shifts though the findings vary widely across studies.
A 6-year study on risk of cardiovascular disease followed 1806 industrial workers and compared the risk factor among shift workers and daytime workers. The life style, serum lipid levels and blood pressure were assessed .The relative risk of coronary heard disease among the shift workers compared to among day workers was found to be 1.4 after adjustment for lifestyle and physiological factors. Increased cigarette smoking and body weight associated with shift work were found to conspire to increase the risk for CHD[ CITATION LTe97 \l 1033 ]. A year later another study found that the risk for CHD increased with longer periods of shift work indicating a dose-response relationship between the duration of shift work and CHD risk (Tenkanen, Sjoblom, & Harma, 1998). These studies agreed with earlier studies involving 79,109 female nurses[ CITATION Ich95 \l 1033 ].
Shift work and stroke and sleep disorder
Several studies have demonstrated the association between shift work and ischemic stroke. The challenge in studying this association is the interaction of stroke and heart disease and the fact that the condition often develops at an advanced age by which time most employees prefer day work due to the inability to cope with the challenges associated with shift work. The exact etiological association between ischemic stroke and shift work is still unclear and there have been many studies to explore the possible mechanism.
A recent study involving 80,108 nurses found that of the 60% of these nurses that had encountered at least one year rotational shifts 1660 cases of ischemic stroke had been reported. The study indicated a dose-response relationship between the ischemic stroke risk and the duration of shift work. The study found that the risk of ischemic risk increased by 4% with every five years of rotational shift work after adjusting for multiple cardiovascular risk factors[ CITATION Dev09 \l 1033 ].
The most definite and inevitable outcome of shit work is a disturbance in sleep patterns. In rotational shift workers the body system has to keep on adjusting to new sleeping patterns. Night shift work means a reversal of the normal sleeping pattern leading to various sleeping problems such as reduced sleeping hours, delayed sleep onset, sleepiness and many others. The sleep problems are also associated with the nature and magnitude of the shift work. Shift work also affects family and social life reducing the support system. The combination of the effects of shift work on family and social life and the fatigue resulting from the sleep problems contributes to psychological problems such as anxiety, stress and depression. These psychological problems often interact with physiological disturbances to cause disease or aggravate existing diseases[ CITATION SLa02 \l 1033 ].
It is also worth noting that the mental fatigue, psychological as well as physiological problems associated with shift work have been be found to have negative impact on workers performance and productive. Shift work and particularly night shift has been linked to increased incidences of work related injuries.
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