Investigation of the effect of unusual work shifts and sleep deprivation on cognitive performance in workers in the automotive industry

Background and aims: The circadian rhythm is one the most important biological rhythms that regulates the sleep-wake cycle and repeats every 24 hours roughly. This 24-hour cycle includes physiological and behavioral rhythms like sleep cycle. Any disruptions in the body’s natural rhythms can cause many problems, such as drowsiness, sleep deprivation, reduced physical activity, and mental functions. Working at unusual clocks is one of the main factors that can interfere with the circadian rhythm. It is estimated that almost 25% of employees have experienced some kind of shift around the world. Studies have showed that about 62% of these shift workers suffer from sleep disorder, which is defined as a general or partial deprivation of sleep. Various studies have also shown that in the night shift, the performance of individuals is reduced by 10-5%. If the night shifts when accompanied by sleep deprivation on the next day, the performance of people is reduced by up to 30%. There are several work shift schedules in different industries. These include fixed shift, split shift, irregular shift, rotating shift and so on. Rotating shift follows an especial speed and direction. The speed means the number of times working a particular shift before moving or rotating to a different one. Working in every morning, evening, and night shifts for two consecutive days. It is necessary to consider one or two days off between the shifts. Employees must at least eight hours free from work when the shift is changed. In some factories, there is no rest day between the shifts. It makes the workers deprive sleeping for one night. It is well known that lack of rest can effect on the human health adversely. Cognitive performance is one of the main functions affected in this situation. Memory, accuracy, and attention as well as reaction time are important cognitive functions which are affected in sleep deprivation. Memory is defined as the ability to record, remind, memorize, and retrieve information. One of the functional differences of the memory system is its differentiation into short-term and long-term memory. Short-term memory is responsible for storing information that is instantly used. Recently, short-term memory has found another definition called work memory. Working memory is actually the new and dynamic of memory, which is actively defined when it is necessary to perform a task. In the most conducted studies, this component of memory has been considered to evaluate the effects of sleep. Many laboratory and field studies have been conducted on the long-term effects of sleep deprivation on cognitive functions but a few studies have been focused on short term effects of sleep deprivation on cognitive functions. Considering the known effects of sleep on physical and cognitive performance of individuals, a study in this field is necessary to make suggestions for modifying such unusual work programs. In Iran most of workers in main industries are in a shift work. Shifts in these industries are often forward rotating. In many of these industries, people are resting for one or two nights after night shift, which makes the person adapt to this situation. However, some factory has an irregular shift work which expose workers with many problems. Therefore, this study was aimed at investigating the short-term effects of sleep deprivation due to unusual shift works on cognitive function of workers in a painting industry. Method: This cross sectional was conducted on the total of workers from painting section of an automotive industry. Considering the inclusion and exclusion criteria 80 persons were selected to the study. The mentioned criteria were alcohol consumption, psychiatric disorders and traumatic brain injury, using psychotropic medications and suffer from epilepsy disorder. Moreover, because the educational status of individuals is one of the factors affecting the cognitive function, all who entered the study had diploma. At first, the baseline and demographic data included age, working experience, quantity of sleep during the previous night shift were asked and recorded. Working shifts were rotating forward from morning to night and night to morning. In this factory, after the night shift, the morning shift began immediately without any rest. In order to investigate the effect of this irregular work shift schedule, cognitive performance was measure in two different conditions. Generally, the study was designed in two phases. Firstly, in the first day of the morning shift following the night shift people who didn’t take any rest in previous night were entered into the first phase of the study. Thus after giving a questionnaire contained some question about quality and quantity of sleep, people with the sleep duration less than two hours were selected for the rest of the study. Cognitive performance was evaluated in three different domains including working memory, selective attention and reaction time by the computerized tests. For assessing the working memory, a validated Persian test of N-Back was applied. STROOP test and Simple Reaction Time Test (SRTT) were also used to measure attention and reaction time status, respectively. In N-Back test, subjects have to react to all of the stimuli so it requires constant control and updating of the information in the working memory and has a very strong reputation for evaluating the working memory. This test has three different levels of complexity. In this study, its first level, N1, was used because studies show that its first level is more sensitive to the effects of sleep deprivation. In this test, 120 stimulants appeared on the screen in 5 minutes. At the first level, the subject had to compare each number with the preceding number and press a certain key on the keyboard. The Persian version of the Stroop computerized test was also used to evaluate the selective attention of individuals. The Stroop color test enables the individual to process related data, thoughts or actions while rejecting unrelated or inaccurate information. In this experiment, four colored circles below the color words are displayed. The person must press the color key word that may not match the color of the circle, regardless of the color displayed on the keyboard. Finally, the error rate and response time are reported as variables in this test. In SRTT, green circles appeared randomly on the screen, which the person had to press a key on the keyboard in the shortest time possible after it appeared. The reaction time was recorded in milliseconds. All tests were carried out in a quiet place where there was no noise, within beginning of morning shift until noon. One week later, the second phase of the study was performed in the afternoon shift when workers got adequate rest in the previous night. It should be noted that in this stage, people were also asked about duration of sleep in the previous night and those who had any sleep deprivation for any reason were temporarily set aside and, at an appropriate time, repeated the tests. The SPSS version 22 was used to stablish the statistical analysis. The tests used included one-way ANOVA, paired t-test and Pearson correlation analysis. Results: The results of one-way ANOVA showed that there was no significant difference between age and work experience (p-value> 0.05). The amounts of sleep duration of people with adequate rest and without any rest were 7.5± 2.5 and 3.5 ± 1 hour, respectively. The mean and standard deviation (SD) of the working memory (N-Back) in both situations, when workers had adequate rest and without any rest were 100.12 ± 5.79 and 85.23 ± 23.13, respectively. The average of obtained scores for selective attention for both situations was 2.51 ± 1. 3 and 8.27 ± 4.4, respectively. Mean and SD of the reaction time test were also 500.21 ±89 and 445.54 ± 105, respectively. Paired t-test analysis revealed that obtained scores for working memory and selective attention, when workers were in the morning shift and had sleep deprivation, were weaker than the time that they had enough sleep in the previous night (p-value< 0.05). Additionally, the level of error in the test was higher in these subjects. The reaction time was also longer in people who with adequate rest time, although there was not a significant difference between two groups. Pearson's analysis also revealed a strong correlation between the sleeping hours and memory scores, as well as a fairly good correlation between sleep hours and the number of errors in the attention variable (r2 = - 0.44). Conclusion: People working in the painting sections of the major automotive industry need to be in an acceptable cognitive performance due to the nature of their work. In general, unusual shift works and sleep deprivation reduce cognitive function in the memory and selective attention scopes because of inadequate rest time between shifts. It has been shown that brains forehead which is responsible for executive functions, shows a significant decrease in metabolic energy consumption in sleep deprivation. Since working memory and attention are both related to this part of the brain, therefore, they are vulnerable to sleep deprivation and change. The results of this study also showed that work in the night shift can make people fatigue, which reduces the executive and cognitive performance of individuals. Therefore, it seems to be necessary adjusting the work schedule in order to considering adequate time to rest between two shifts in unusual work systems.