Competitive athletes are placed under substantial pressure to perform at their best due to considerable advancement in sports science support, technology and rewards for medal positions from competitions [40]. It is therefore crucial that any potential enhancement in performance which may provide a competitive edge is thoroughly explored. Multiple studies have uncovered a link between chronotype and physical performance enhancement [20, 21, 41, 42], supporting the idea that time of day and individual differences in circadian timing may impact diurnal variation across other measures of performance [43]. However, to our knowledge, this is the first study to use multiple indices of cognitive and physical performance in regard to chronotype and time of day.
Using a multifactorial approach to explore the different factors and variables which may affect enhancement in performance, our results showed significant differences between ECTs and LCTs in sleepiness, as well as across multiple cognitive performance measures, e.g. reaction time (PVT)and executive function (EF). Furthermore, we show that a simple physical performance measure (maximal voluntary contraction of isometric grip strength), exhibits a significant diurnal variation between the two groups. This is consistent with the results on a more complex measure of physical performance, namely, cardiovascular endurance [21]. Importantly, our results uncovered that ECTs performed significantly better than LCTs across all performance measures in the morning (08:00 h). The morning testing session took place when LCTs would usually be sleeping, which supports previous research claims that completing tasks during one’s ‘biological night’ can be detrimental to performance [44]. This finding has since solidified the importance of chronotype identification within athletes.
Chronic misalignment is generally associated with differences between an individual’s endogenous circadian system and external time cues [45]. Typically, LCTs experience chronic misalignment due to following an earlier schedule during the ‘work week’ and reverting back to later sleeping patterns on ‘free days’. Commonly known outcomes of this circadian misalignment, such as jet lag or night shift work, are known to negatively impact on health and performance [46].
Time of Day, Chronotype and Daytime Sleepiness
Our results showed LCT’s subjective sleepiness at 08:00 h (Fig. 1) was significantly higher than ECTs at the same time, with a three-point difference on the KSS to distinguish between ‘alert’ and ‘some signs of sleepiness’. This could be an important consideration for LCTs, who often have to ‘perform’ before their entrained/habitual wake-up time. This is particularly important in professions such as pilots, medical professionals, military personnel, commercial drivers and other occupations whereby a reduction in alertness and decision-making capabilities may be consequentially life-threatening [47]. This desynchronization is also prevalent within an athletic environment, whereby an athlete may be required to travel across time zones to compete. If the misalignment is not corrected prior to competition, an athlete’s decision making, alertness and executive functions may be hindered, resulting in non-optimal performance [48].
Time of Day, Chronotype and Measures of Performance
Optimised cognitive abilities are essential to the basic functioning and have also been recognised as an important component of successful athletic performance [49, 50]. PVT reflects the attentional state of an individual [51], and research has shown that a better PVT score is associated with improved response time and accuracy in interceptive sports, such as tennis and squash, as well as improved response accuracy in strategic sports such as field hockey and soccer [52]. Executive function incorporates cognitive factors including working memory, problem-solving and decision-making, taking place in the prefrontal cortex [53]. A significant correlation has been found between the level of sporting ability and success rate in completing executive function tasks [54]. Further research has also shown that self-paced athletes, such as swimmers and runners, perform better at inhibition tasks. In contrast, externally paced sportspeople, such as rugby and soccer players, score higher on problem-solving tasks [55]. As a result of these findings, it is clear that the multiple aspects of cognition are imperative to an athlete in order to achieve maximum and well-rounded performance.
One of the key findings from this study highlights that cognitive performance is significantly impaired in LCTs when they are required to perform both simple and complex tasks during the morning. We have shown that LCTs are compromised at 08:00 h in both the PVT and EF tasks, with performance being significantly worse than ECTs by 8.4% and 5.9%, respectively. Coupled with the significantly higher ratings of sleepiness at 08:00 h, this is consistent with research that partial sleep deprivation can result in an increased response time and higher number of lapses when undertaking the PVT [33, 56]. Interestingly, there were significant diurnal variations in PVT performance for LCTs but not for ECTs. However, when looking at a more complex measure of cognitive performance during the EF task, this relationship was reversed. Although a non-significant but gradual improvement was seen across the three testing sessions in LCTs, it was only ECTs that showed significant diurnal variations (Fig. 3). A potential reason for this could be attributed to the complexity and nature of the task. Two recent meta-analyses have shown that sleep deprivation has greater negative impact on the performance in simple cognitive tasks, such as the PVT, compared to more complex cognitive tasks [57, 58]. Both papers attribute this to the higher degree of boredom and lower arousal that is associated with simple tasks and suggested that these factors may be amplified due to a lack of sleep. More complex tasks, however, require and generate greater engagement and stimulation, therefore potentially accounting for the detrimental effects of sleep loss on performance. Our results show that LCTs have a damped amplitude in diurnal variation for the EF task, suggesting a potential compensatory reaction during a more complex task. This finding aligns with the research that suggests simple tasks are more adversely affected by sleep deprivation [57]. The fact that the amplitude of diurnal variations seems to be impacted by the nature of tasks, i.e. simple vs complex, as well as between chronotypes, presents an interesting area for future research.
It is well established that elements of physical performance, particularly those involving muscular strength, tend to peak in the early evening [18]; however, much research has failed to consider the impact of chronotype. Our study showed that ECTs performed their best MVC at 14:00 h, whereas the peak for LCTs was at 20:00 h (Fig. 4).
Performance as a Function of Time Since Entrained Awakening
Time since entrained wake-up has been proposed as a predictor of peak performance in aerobic endurance tasks, with the peak for LCTs occurring significantly later compared to ECTs [21]. This finding is consistent with the measures of performance shown in our study in healthy volunteers. The main observations relating to chronotype and performance since the time of awakening are (1) peak performance in ECTs always occurs closer to the habitual wake-up time compared to LCTs. (2) For ECTs, measures of cognitive performance are best almost immediately after entrained wake-up time, whereas measures of physical performance peak between ~ 5/6 h and ~ 7 h after wake-up time for aerobic [21] and MVC, respectively. (3) Regardless of the measure of performance, LCTs do not reach their peak until at least 12 h after entrained wake-up time. This suggests that LCTs have a much narrower window of opportunity to perform at their best during the course of a typical day, which could have significant implications for athletes with a late chronotype who are required to train and compete earlier than their biological peak.
Our findings support much of the current literature by suggesting clear differences in performance profiles between ECTs and LCTs. These results complement those previously recorded with more complex measures of physical performance [20, 21, 42] by showing similar trends of significantly better performance from ECTs in the morning, as well as very different peak times as a function of time since entrained awakening.
Implications for Performance
Knowledge of the potential impact that an individual’s chronotype may have on both cognitive and physical performance could have significant beneficial implications within the general population. This information could also provide new insight to sectors that require individuals to achieve optimal performance such as military personnel, first responders (firefighters, police and paramedics) and professional athletes. At an elite sporting level, a winning margin can be as little as 1%. At the most recent 2016 Olympic Games (Rio de Janeiro, Brazil), had the fourth place swimmer in the men’s 100 m freestyle improved his time by 0.5% (0.24 s), it would have been enough to secure a gold medal. Similarly, if the last placed competitor in the men’s 100 m sprint had run 0.25 s faster (2.5%), it would have been enough to beat Usain Bolt. These differences are so minute that any potential advantage to be gained should be researched in depth. Previous research has shown that aerobic performance can vary up to 26% over the course of a day [21]. The present study supports this by showing a ~ 10% diurnal increase in a simple measure of physical performance and ~ 9% and ~ 7% variation in simple and complex measures of cognitive performance, respectively. Using these findings, training strategies could be developed and implemented by coaches to maximise performance through adhering to the athlete’s individual chronotype and taking into account the time since entrained awakening. This would be of particular relevance to LCTs who have been shown to exhibit greater variation in diurnal performance profiles. If LCTs are required to train/compete during non-optimal morning hours, it could significantly influence their abilities since our study shows that LCTs are compromised at 08:00 h. These results do not just apply for individuals but also teams, as previous research has shown that chronotype distribution within a team is highly predictive of overall performance, highlighting how this research can be used to give teams a ‘circadian advantage’ [41]. However, rigid schedules often present a challenge for athletes having to perform at non-optimal times. Therefore, this information could be used to develop personalised interventions targeting at sleep and circadian biology aimed to shift the timing of peak performance to accommodate inflexible competition times. It is fair to speculate from our results that such a strategy may result in greater improvements in overall performance, although in-depth research is required to develop these approaches.
Limitations
The purpose of testing at clock times as opposed to internal biological time was to investigate how these two groups behave during the hours of a ‘normal day’ (08:00 h to 20:00 h). This data can therefore be translated into real-world settings and hold implications for monitoring performance. The disadvantages of this design are that it cannot separate the number of influences affecting the outcomes or separate the effects of the circadian system from the sleep homeostat. To explore truly circadian effects, strict laboratory-based protocols, e.g. constant routine or forced desynchrony, over 24 h or more would be required. However, it could be argued that the combined approach is more applicable to the real world as behaviour and performance are ultimately impacted by both factors, and the more controlled protocols could result in poorer external validity due to the unrealistic settings.
Here, we investigated a relatively simple measure of physical performance using isometric grip strength, and thus, we restrict the ability to determine how more complex measures would be affected [31]. The performance itself is multifaceted and cannot be defined by one mechanism alone. Internally, physical performance can be measured by physiological markers such as hormones levels (e.g. cortisol, melatonin, and testosterone), CBT, heart rate, respiratory rate and maximum oxygen uptake. External physical performance can be described and monitored through strength, power, aerobic capacity, anaerobic capacity and specialist skills tailored to certain sports such as accuracy. These varied and complex processes cause the research of ‘performance’ to be viewed differently by clinicians, psychologists, sports scientists and others.
The sample population was predominately either competitive athletes or individuals who engaged with sport on a regular basis (84% of ECTs and 81% of LCTs). Therefore, we believe that the sample studied here is a representative cohort for the athletic implications that we suggest in the manuscript. The choice of not seeking to recruit specifically elite athletes was made on accounting for practical difficulties in doing so, but more importantly in order to allow the best possible compliance and accurate, reliable data collection. The reality of the daily schedule of elite athletes would present many constraints that would limit the ability to carry out this study design accurately. For example, athletes tend to follow very strict training and competition schedules, which do not allow them to sleep/wake according to their biological preference. All participants in this study followed their preferred routines for the duration of the testing period, thereby allowing us to gain a true indication of their chronotype without ‘masking’ effects of imposed schedules. The study design also required participants to provide saliva samples and attend the laboratory for multiple testing sessions, something that would be difficult for elite athletes to comply with. Carrying out real-world research in healthy volunteers is necessary in order to inform populations such as elite athletes; but also, because this study design investigates elements that contribute to athletic performance and not athletic performance per se, the findings of the study are relevant for a wider range of physically active individuals.
Therefore, it must be acknowledged that the measures tested and the sample population used in this study could restrict the ecological validity. However, our results show a strong similarity to previously published work on aerobic capacity in athletes [21], and previous studies have shown measures of muscle strength do correlate with sprint and jump performance [59].
