The influence of sleep, caffeine, time of day, tiredness on workplace productivity and cognitive function

Introduction

Organisational psychology greatly influences the health of the organisation, its employees' well-being, and society's functioning(Martin et al., 2022). The notion of productivity is often associated with economic performance and job contentment. Still, it encompasses personal well-being and the well-being of a given organization (Redeker et al., 2019). For instance, earlier studies have demonstrated that enhanced productivity can improve mental and physical health, decrease stress levels at work, and improve the organisation's productivity.

Nonetheless, productivity is not a simple concept. It arises from the interplay of multiple aspects that include biological, psychological, and environmental characteristics (Hller et al., 2021). Comprehending these factors is essential for organisations to enhance employee output and satisfaction. Sleep, tiredness, time of day, and caffeine influence cognitive capabilities such as attention, memory, and decision-making making which are also important cognitive functions commonly utilised in the workplace (Bufano et al., 2024; Ricupero et al., 2024)

Current Understanding

Sleep is a biological behaviour, and there is a consensus that sleep greatly affects cognitive processes, especially those that involve attention, encoding, and memory consolidation (Diekelmann, 2014). Effective information processing and the capacity to stay focused on a particular activity are adversely connected with sleep deprivation and positively associated with sleep, as demonstrated by Tai et al. (2022).

Tiredness is the feeling that one requires rest or sleep, which can be attributed mainly to long periods of wakefulness and mental and physical exertion and has been found to have adverse effects on cognitive performance over extended periods. In its most general sense, it hinders attention and interferes with decision-making, problem-solving, and other mental activities, thus reducing productivity or efficiency. However, it is essential to comprehend that fatigue and tiredness are different. Fatigue is a more chronic problem, whereas tiredness can be resolved with a whole night's sleep. Additionally, this research focuses on subjective tiredness, specifically how tired individuals feel at the time of testing.

Considering the time of day (ToD), there are performance cycles (natural variations in cognitive and physical performances, such as energy levels) related to circadian rhythms. These performances peak twice during the day frequency, most likely mid-morning and early evening, but this also varies across individuals (Teo et al., 2011). There have been documented effects of ToD on performance, work output, and the alertness of people. For instance, surgeons favoured operating between eight in the morning and twelve in the afternoon because this time frame coincided with their times of highest alertness (Arab et al., 2022). Yet, the literature is very sparse in detailing the effects of changes in the rhythm of other variables, for example, quality of sleep and caffeine levels among others on these other variables in real-life settings.

Following this, caffeine is a central nervous system stimulant that momentarily enhances concentration and alertness (Chen et al., 2020). Although studies have demonstrated that caffeine consumption enhances performance at work (Smith, 2005), the topic of caffeine continues to be controversial. Zhang et al. (2020) discovered that while greater dosages (6 and 9 mg/kg) of the stimulant did not boost cognition, precisely reaction time assessed by the Stroop task and utilised a congruent and incongruent condition, low levels of 3 mg did. Smith (2002) accentuates the contention that caffeine affects individuals variably since tolerance levels vary and the metabolism of the stimulant fluctuates by body weight.

The Role of Sleep in Cognitive Function and Workplace Productivity:

Sleep assumes an important state of memory consolidation, sustained attention, and executive functioning, as made known by De Bruin et al. (2017). The Two-Process Model of Sleep Regulation addresses this by putting sleep homeostasis (Process S) and circadian rhythms (Process C) in opposition or synchrony. The arrangement of sleep and waking states in time relates to Process C, while Process S is related to positive sleep pressure and recovery, which rises during wakefulness and falls during sleep (Borbly, 1982). Cognitive functions like attention, memory, and decision-making can be severely hampered when any processes are disturbed, such as by inadequate sleep or circadian misalignment (Borbly, 2022).

Sleep is essential for cognitive functioning. According to Khan et al. (2023), sleep deprivation is the result of an individual not getting enough sleep for extended periods, which accumulates over time. Due to a decrease in hippocampal volume and a decrease in grey matter in the pre-frontal cortex (PFC), two crucial brain regions involved in memory, attention, and decision-making, sleep deprivation negatively impacts cognitive functioning (Krause et al., 2017). Sleep deprivation differs from regular sleep loss as it is more prolonged. This current study aims to examine how differences in the quantity and quality of sleep on a typical day, such as getting less sleep than usual one night, affect cognitive function and workplace productivity. As sleep is essential for day-to-day functioning, lack of sleep leads to a decline in cognition, which becomes an issue when these same functions are required during work, which in turn affects workplace productivity and leads to more errors, reduced reaction time, and poorer vigilance (Pilcher & Morris, 2020).

To examine the association between presenteeism, sleep, and job productivity, Ishibashi & Shumara (2020) gathered 2987 participants for their study, of which 1835 were men and 1062 were women, ages 18 to 76. Participants were drawn from various industries, including music, fashion, healthcare, and trade. They discovered that presenteeism among those who slept less than five to six hours decreased their productivity. The Pittsburgh Sleep Quality Index (PSQI) and the Short Form of the Work Limitations Questionnaire (WLQ-SF), two clinically validated instruments used to evaluate sleep quality and the effect of health issues on work performance, were employed in their methodology (Mollayeva et al., 2016; Walker et al., 2017). The two-process model of sleep provides a useful framework for interpreting these results. This is because insufficient sleep, which is highlighted through their research disrupts process S, and process C due to the misalignment of their normal circadian clock. Although their study's sample size was large, which is creditable as it enhances the statistical power and the generalisability of this research (Lakens, 2022) and used validated tools, which increases the scientific tenability of the study, this research was focused on sleep concerning health problems, and workplace productivity not solely workplace productivity nor cognitive function, which further exacerbates the dearth of literature in the realm of sleep and its effects on workplace productivity and cognitive function integrated with caffeine, time of day and tiredness which is what this research seeks to address.

The fact that their study was conducted in Japan should also be underlined. Existing research has already established shorter sleep duration and longer work hours in Asian countries (Dunleavy et al., 2019). As a result, the findings' applicability is limited due to the differences in sleep duration and quality in the United Kingdom. This highlights the absence of research that can be broadly generalised in the UK, particularly for workplace productivity and cognitive function.

Yang et al. (2018) evaluated the relationship between sleep and productivity at work. They discovered that long and short sleep durations were linked to decreased productivity, which was assessed using the Well-being Assessment of Productivity. Sleep duration was categorised as any sleep duration less than four hours, considered very short; five to six hours, considered short; seven to eight hours, considered normal; and nine hours or more, considered long. In addition to using the Epworth Sleepiness Scale (ESS) and 1007 participants aged 22 to 60, they examined several characteristics, including snoring, insomnia, and tiredness. While they found an association between sleep duration (very short, short, and long) and lower productivity, insomnia accumulated the most significant impact on work productivity. Their research was crucial to comprehending how sleep duration impacts productivity at work, as Lanaj et al. (2014) discovered that a reduction in sleep quantity reduced productivity at work, notably in their study on smartphone use. They evaluated the individual's smartphone usage the night before work and its impact on the following workday. The findings depicted that employees who used their phones within an hour of bedtime suffered from shorter sleep duration, which triggered morning depletion and decreased productivity the following day.

Although all of these studies are pertinent and advance the comprehension of the significance of sleep quality in organisational psychology, more research is needed to assimilate how sleep impacts workplace productivity while also taking into account variables that are common in daily life, such as caffeine, the time of day, and tiredness. Yang et al. (2018) and Lanaj et al. (2014) do not address these issues. Although they all examine factors including smartphone use, sleepiness, and snoring, their focus on sleep is informative. To produce more generalisable findings about how commonplace elements like the time of day, caffeine, and tiredness also affect workplace productivity and cognitive functioning, this study aims to address the gap in the literature by providing students and the general public with the opportunity to participate. Additionally, none of these studies directly evaluate sleep at the time oftesting, which is also the objective of this study. It seeks to determine how sleep, tiredness, time of day, and caffeine impact cognitive functioning and workplace productivity throughout the night before or the morning of testing.

Sleep quality:

Peng et al. (2023) found a significant association between poor sleep quality and occupational well-being with the partial mediating role of self-efficacy. They used 487 participants (309 male and 178 female) aged between 24 and 46 from different occupations, such as food and logistics transportation. They applied the PSQI to assess sleep quality, the Occupational self-efficacy scale, and three subscales to measure occupational well-being. While their findings expanded the scope of research and how employers should consider employees' sleep quality to optimise workplace performance, they focused on occupational well-being and self-efficacy, not workplace productivity. According to Putra et al. (2024), occupational well-being is essential as it influences workers' motivation and job satisfaction; however, evaluating workplace productivity differs since it centres on performance, which the present research attempts to do. Caffeine, sleep, tiredness, and ToD were not considered in Peng et al.'s (2023) study. This literature review has demonstrated that these factors affect workplace productivity, which is why this study seeks to determine how these variables affect cognitive function and workplace productivity. Also, Peng et al.s (2023) findings were reiterated through Howe et al. (2024) who similarly found poor sleep quality was associated with work-related burnout, personal burnout and psychological distress. These factors are essential components of occupational wellbeing and can affect ones workplace productivity.

Impact of Tiredness on Workplace Productivity and Cognitive Functioning:

According to Pilcher and Morris (2020), physical and mental tiredness influence performance adversely in the workplace, decreasing arousal, slowing reaction times, reducing motivation, and increasing distractibility. From a psychological viewpoint, it comes with a drop in motivation and an increase in one's distractibility, both counterproductive. In addition, these effects are more severe when the tasks performed require prolonged attention or high-level cognitive tasks such as problem-solving and decision-making. As Lerman et al. (2012) explain, tiredness is the body's response to sleep loss and it can reduce an individuals vigilance, reaction time, and loss of awareness during critical situations. Parent-Lamarche and Marchand (2018) found tiredness with lower output and higher error rates in office settings. However, their study was more self-reported fatigue than an objective measurement of the sources and sectoral distinctions. In a healthcare setting where life-or-death decisions made with fatigue could vary immensely, the previously described dynamics do not hold in a learning or corporate environment (Pignatiello et al., 2020).

The Stanford Sleepiness Scale (SSS) is one of the most regularly used tools for measuring subjective tiredness, and the higher the score on this scale, the lower the performance level in the tasks assessed (Li et al., 2016). However, this is a problem as most of the studies that use this approach ignore entirely the problem of tiredness due to other factors (Martin et al., 2022). For example, subjective measures allied to cortisol level can be viewed as an undistracted objective marker of physical tiredness and tend to be seldom used due to practical issues. Arguably, sleepiness may be a potential indicator of tiredness, and as Pilcher and Morris (2020) portray, the SSS can provide useful insights in workplace contexts which is why this research is also utilising it.

The volume of research on the impacts of fatigue on cognitive function and workplace productivity is also noteworthy, as is the paucity of research on the effects of tiredness on workplace productivity. According to Sadeghniiat-Haghighi et al. (2015), psychologically, fatigue refers to a lack of energy to perform tasks and affects one's ability to function at work. Similarly, Lerman et al. (2012) described how fatigue in the workplace can lead to a decrease in vigilance, a loss of awareness, and slower reaction times. They also focused on shift workers, particularly healthcare and transportation workers who work night shifts, and found that they are more fatigued and have lower cognitive function and workplace productivity. Since there is little research on tiredness and its effects on these factors, this study seeks to address that gap in the current literature.

Furthermore, the terms 'fatigue' and 'tiredness' are used interchangeably in research. For instance, Fan and Smith (2020) investigated the effects of occupational fatigue on the cognitive functioning of nineteen participants, fourteen of whom were males. They materialised a diary and two online cognitive tasks: a logical reasoning task and a visual search. Participants had to complete the diary and cognitive tasks before and after finishing work on the first and last days. Although their findings demonstrated an association between occupational fatigue and decreased cognitive functioning, specifically decreased vigilance, reaction times, and accuracy, it is hard to establish whether the findings also apply to tiredness and the distinguishment between the uses of the terms throughout their research. According to Phillips (2015), fatigue is a chronic condition, while tiredness is a daily occurrence that may be overcome. However, the uncertainty between the two definitions makes it difficult to identify which term researchers use in their studies. This emphasises why this study aims to specifically examine subjective tiredness in conjunction with the other variables and its effects on workplace productivity and cognitive functioning.

Time of Day and Cognitive Performance and Workplace Productivity

In circadian cycles, cognitive performance varies across the day. A chronotype refers to a person's endogenous circadian clock, which is influencedby internal and external variables and controls behavioural and physiological processes such ashormones, sleep, and cognition (Chauhan et al., 2023). An individual's optimum performance peak is related to their chronotype (Valdez, 2019). Morning types ("larks") are best in the early hours, while evening types ("owls") are at their best later (Pardo et al., 2020). Schmidt et al. (2015) study comprised 32 participants, ages 19 to 30, 16 of whom were morning types and 16 of whom were evening types and assessed cognitive performance using fMRI scans and working memory N-back tasks that recalled various stimuli. During their research, they employed the PSQI, the ESS, and the Morningness-Eveningness Questionnaire (MEQ). As a result, they discovered a higher thalamic activity when evening chronotypes completed the high working memory load task as opposed to the morning chronotypes, who exhibited higher activity in the middle frontal gyrus through the same memory task in the morning.

Chronobiological research on the effects of ToD on performance suggests that their impact has important consequences regarding employees' productivity (Gabay et al., 2022). Remembering things, processing information, and coming up with solutions to problems are the kinds of work performed best during the individual's affluent part of the day. Problems arise when employees are expected to engage in these otherwise productive activities at other times, and this is mostly the case when the preferred work time cannot be accommodated.

The impact of chronotypes and the ToD on creativity was examined by Khnel et al. (2022). The 260 participants in this repeated measures field study were 51?male, had an average age of 35, and were divided into two groups based on their professions: engineering, healthcare, retail, finance, and education. The chronotype was evaluated using the Munich chronotype questionnaire by allocating the first group to attend morning or evening classes at their workplace and the second group to complete two online questionnaires during their work hours, one in the morning and one in the evening. Their findings corroborated the synchronisation effect between chronotypes and circadian cycles improving cognitive performance, which was also established by Schmidt et al. (2015). Although their results were significant, the primary emphasis of this study was only creativity. Creativity is only one component of cognition, and this study ignores other cognitive domains like memory and decision-making. Also, the PSQI was used to measure the control variable sleep in this study, which, for the purpose of their research, was satisfactory; nonetheless, the relationship between sleep and ToD cannot be overlooked as they are interrelated. For this reason, this research aims to address the effects of ToD alongside sleep and other factors to gain a more thorough understanding of cognitive functioning and workplace productivity.

Alternatively, Kiema-Junes et al. (2023) offer proof of the discrepancy between a person's chronotype and work schedule. In their analysis, they identified how healthcare workers who work night shifts experience a decline in their cognitive performance, which leads to errors during work and lower workplace productivity due to them having a morning chronotype, and conversely, how daytime workers also suffer a decline due to them having an evening chronotype and in both cases, working away from their preferred ToD results in lower cognitive performance, lower workplace engagement, and a mismatched circadian rhythm. They also employed the MEQ to determine the chronotype of participants, a validated tool being employed in this study, which is beneficial as it offers a strong basis for determining participants' chronotype and comprehending its direct correlation with cognitive functioning and workplace productivity (Horne & Ostberg, 1976). Additionally, the MEQ evaluates sleep indirectly based on work patterns that are not aligned. Given this, Kiema-Junes et al. (2023) overlook the interrelation between sleep and preferred ToD, highlighting the need for studies like this one to look at the effects of sleep, ToD, caffeine, and fatigue collectively rather than separately on cognitive functioning and workplace productivity. Since employers are prone to ignore these common factors when evaluating workplace productivity, this study attempts to assess them collectively and offer employers insight on optimising employee efficiency in organisational environments.

Caffeine Consumption and Cognitive Function and Workplace Productivity

Caffeine is a common stimulant in food and beverages that enhances cognitive functioning(Bessada et al., 2018). By functioning as a competitive antagonist in the central nervous system, it blocks adenosine receptors, mainly A1 and A2A receptors, and is a psychoactive substance (Fiani et al., 2021). Adenosine is a nucleoside that regulates sleep, memory, cognition, and neurotransmitters like glutamate and dopamine (Sheth et al., 2014). Caffeine temporarily increases alertness, arousal, and reaction time by inhibiting this (Pasman et al., 2017).

Arieputri et al. (2018) evaluated the effects of caffeine (113.5mg) on accuracy and response time in prolonged attention tasks through experimental workouts. The study used a control group design and an experimental methodology to examine the effects of caffeine consumption on college students' attentiveness, which was measured using the correct answers on the Stroop Effect Test. 36 Indonesian first-year psychology students, aged between 18 and 19 years old, 30 female and six male, were split into experimental and control groups. Caffeine did not significantly affect attention levels after adjusting for participants' sleep duration, which was attributed to the conclusion. This suggests that other factors may influence cognitive performance, highlighting the novelty of this research, which examines the effects of caffeine on cognition and workplace productivity in tandem with time of day, tiredness, and sleep.

Moreover, Vital-Lopez et al. (2018) revealed caffeine's transient effects, claiming it offers context-related benefits. For instance, Schweitzer et al. (2006) found that napping and caffeine together boosted the cognitive function of 53-night shift workers. To assess performance and alertness, they employed the maintenance of wakefulness test and the psychomotor vigilance task. Their results emphasise the need for research such as this to examine the effects of sleep and caffeine with other variables on cognition and productivity, as it is evident that there is an interplay between sleep and caffeine. On the contrary, though caffeine at mild and moderate dosages, between 20 to 80mg, enhances performance, Lui et al. (2024) exclaimed how doses over 400mg of caffeine can lead to paradoxical effects, such as heightened anxiety and reduced concentration in an individual (Lui et al., 2024). Considering this literature, caffeine's effects are not considered in workplaces when attempting to maximise workplace productivity. For this reason, this research is crucial as it attempts to identify the effects of these commonplace variables on work productivity.

Longer hours spent at a desk can be inhibiting, lowering creativity levels, mishaps in decision-making, and burnout. A moderate amount of caffeine (40200 mg) can aid short-term alertness but may also reduce overreliance. This viewpoint is challenged by Herqutanto et al. (2024), who used 121 Indonesian office workers to test the alertness of coffee drinkers and non-drinkers and found no discernible differences in alertness between the two groups before or after coffee consumption. Although their study did not produce enough results, it facilitates additional research. The sample size is inapplicable to the workplace in non-Western nations since, according to Jannah et al. (2023), the average amount of caffeine consumed in Indonesia is 238 mg. Alternatively, according to dePaula and Farah (2019), the average in Western countries is 138 mg. Thus, coffee intake is higher in Indonesia, which results in mild effects of caffeine on cognitive performance. Herqutanto et al. (2024) found no difference in alertness but only used 72 mg of caffeine. This underscores that Herqutanto et al. (2024) did not adequately address productivity because, as Cappelletti et al. (2015) demonstrated, cognitive improvements depend on the amount of caffeine consumed. This aligns with the inverted U theory of arousal, which posits that performance increases until an optimal point, after which it demerits, which was demonstrated by Doyle et al. (2016), who found after 6mg of caffeine, performance deteriorated. Therefore, this research aims to explore how caffeine affects productivity at the time of testing, similar to Herqutanto et al. (2024); however, with a focus on variables such as tiredness, time of day, and sleep, factors they did not account for in their study. These distinctions and differences between sector-specific features are crucial for better productivity and cognitive benefits in various workplaces in which interventions can be tailored toward improvement.

Combined Effects of Sleep, Tiredness, ToD, and Caffeine

The link between sleep, tiredness, ToD, and caffeine consumption on performance has not received considerable attention, even though studies indicate a correlation between the aspects (Chen et al., 2020). For example, since sleep deprivation has its negative effects, a caffeinated beverage might help to dispel those effects for a short while. Nevertheless, people's preferred time to engage in specific activities, as influenced by their biological clocks, can either increase or lessen such compensatory behaviours. The increasing instances of burnout apparent in today's workplaces provide an extra layer of diagnosis. Also termed chronic stress, burnout involves prolonged periods of tiredness, low work performance, and socio-pathological behaviour that could affect these circumstances. For instance, the Burnout Assessment Tool (BAT) is one of the techniques oriented to measure and study burnout in sustainable workplace research (Schaufeli et al., 2020). This explanation looks very simple because sleep, tiredness, ToD, caffeine consumption, and burnout all together predict productivity well. However, consider the instance of healthcare workers. They are specifically given intervention services based on the effects of burnout-nine chronic stressful experiences with dimensions of exhaustion, reduced self-performance, and disengagement (Schaufeli et al., 2020). Particular attention should be devoted to these relationships when considering the ability to develop polite revolutions and workplace strategies to maintain high performance and health effectiveness. Previous studies shed little light on the understanding of these relations. Evidence from research on caffeine and capacity sleep deprivation indicates that the advantageous effects of caffeine are sometimes greater in those deprived of sleep, but these benefits are time-dependent (Crooks et al., 2019). Similarly, tiredness behaves with a circadian rhythm, and deviant time variations of performance are commonly noted during extreme tiredness periods (Bartrim et al., 2020).

The secondary influence of sleep and ToD on cognitive performance and productivity varies significantly across areas. In education, early school start times would misalign with the chronotype of students, which would mean less attention and less cognitive focus. This is supported by Vinne et al. (2015) study since they found associations between sleep and chronotype variations in students because, for example, a student who had a short amount of sleep (less than seven hours) and a late chronotype, which the MCTQ measured resulted in lower academic achievement. While this study is credible as it highlights the interrelation between the time of day and sleep and offers insights on future implications beneficial for academia and students, it lacks focus specifically on workplace sectors. However, it sets the foundational base for further research such as this study, which aims to focus specifically on how workplace productivity is affected not only by ToD and sleep but also by caffeine and tiredness.

Alternatively, sleep and tiredness are much more exacerbated in healthcare shift workers. Disrupted sleep and perpetual tiredness among medical professionals can result in a longer reaction time, impaired decision-making, and a higher rate of errors in critical procedure conduction (Vlasak et al., 2022). It is crucial to consider how these factors interact, as previous literature lays the groundwork for this study to address the gaps in knowledge about the cumulative effect of sleep, tiredness, ToD, and caffeine on cognitive performance and workplace productivity.

Gap in Literature

While a substantial body of literature highlights the individual significance of various variables, several critical issues persist. Firstly, much of the existing research tends to examine these variables in isolation, neglecting to explore their interactions; for instance, it remains unclear how the benefits of caffeine might be influenced by an individual's sleep quality or circadian rhythms (Mason et al., 2021). Secondly, a significant portion of studies is conducted in controlled laboratory settings, which diminishes ecological validity and limits the applicability of findings to real-world work environments and daily cognitive demands (Beilak et al., 2017).

This review emphasises a need for research that brings sleep, ToD, tiredness, and caffeine into the discussion when considering workplace productivity. Most of the extant studies have been limited to highly controlled environments. Thus, they generally neglect the specific demands from different sectors and their practical applications or either have research done solely on variables such as caffeine (Arieputri et al., 2018) and sleep (Yang et al., 2018) but have not taken sleep, tiredness, ToD and caffeine into consideration collaboratively. Currently, these factors are unlikely to be considered in all workplace sectors. Therefore, this research is for employees to understand and acknowledge the effects of these everyday variables on individuals' workplace productivity and cognitive function. These understandings of variables in their real-world contexts may lead to interventions that improve cognitive outcomes and efficiency within workplaces, especially in demanding sectors such as healthcare, education, and corporations. This opens future research avenues that ideally address the evidence base for productivity and cognition enhancement across various occupational environments.

Current aims and hypotheses:

This research fills a void in understanding sleep, tiredness, ToD, and caffeine on work performance. Following circadian rhythms and regulating sleep through homeostasis, this paper attempts to study their interactions at the time of testing for a better understanding of cognitive ability, as these factors will have the potential to influence workplace productivity and cognitive function; however, they are not considered in these sectors when people engage with the tasks. They influence how well people perform tasks, influencing workplace productivity outcomes.

In light of the previous literature review, these hypotheses have been derived:

ToD hypothesis: Participants who complete the questionnaires further away from their preferred ToD testing will have poorer workplace productivity scores (higher scores on the Burnout Assessment tool) and poorer cognitive functioning (higher scores on the Workplace Cognitive Failure Scale).

Sleep quality hypothesis: Participants who report having higher sleep quality will have higher workplace productivity scores (lower scores on the Burnout Assessment Tool) and higher cognitive functioning (lower scores on the Workplace Cognitive Failure Scale).

Sleep quantity hypothesis: Participants who report extreme sleep durations (participants' duration of sleep less than six hours of sleep) will have poorer workplace productivity scores (higher scores on the Burnout Assessment Tool and lower scores on the work productivity questionnaire) and poorer cognitive functioning (higher scores on the Workplace Cognitive Failure Scale)

Tiredness hypothesis: When taking the test, participants who report higher levels of subjective tiredness will have poorer workplace productivity scores and poorer cognitive functioning (higher scores on the Workplace Cognitive Failure Scale).

Caffeine hypothesis: Participants who have had any caffeine-contained food stuff (CCFS) before testing will have higher workplace productivity (higher scores on the workplace productivity questionnaire and lower scores on the Burnout Assessment Tool) and higher cognitive functioning (higher scores on the Workplace Cognitive Failure Scale).

Stanford Sleepiness Scale (SSS) hypothesis: Participants who score higher on the SSS will have lower workplace productivity and cognitive function.

Interaction effect: There will be an interaction effect between sleep, caffeine, ToD, and tiredness on workplace productivity and cognitive function. However, this analysis remains exploratory due to the lack of literature on these variables.

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