While player fatigue in football is well-documented (Mohr et al., 2005; Bangsbo et al., 2006), little is known about the effect of fatigue on referees’ performance. It was previously suggested that the physiological demands of refereeing are similar to a midfielder, as each covers a comparable total distance with equivalent durations of high-speed running (Weston et al., 2011b). Like players, referees cover 7.5 to 11.5 km per game (Weston et al., 2011b; Costa et al., 2013). Although many studies report total distance covered (Castagna et al., 2007; Mallo et al., 2007; Weston et al., 2011b), this may not be a reliable measure of physical stress given that standing, jogging and walking account for more than 75% of refereeing activity (Weston et al., 2007). Instead, high intensity running (HIR) is a more reliable measure of the physical demands and therefore a better indication of fatigue (Krustrup et al., 2002; Mallo et al., 2009). A high-level football referee spends 42% of the match running at high intensity (18.1–24 km.h^-1), with approximately 89% of maximal heart rate throughout the match (Krustrup et al., 2002; Castagna et al., 2007).

Other authors suggested that comparing the distance covered in each half of a match could reflect the level of fatigue of referees (Weston et al., 2007). While some studies reported a decreased distance covered in the second half (Catteral et al., 1993; D’Ottavio and Castagna, 2001) others showed no significant differences (Krustrup et al., 2002). Similar results were observed when comparing HIR distance between first and second half of a match (D’Ottavio and Castagna, 2001; Krustrup et al., 2002). The variation in the observed results can be related to several external factors such as player tactical roles, players’ physical condition, referees’ stress levels, and environmental conditions (Weston et al., 2011a; Costa et al., 2013). Because of this, it was suggested that the subjective evaluation of fatigue is worth considering for example by using the rating of perceived exertion (RPE; Impellizzeri et al., 2004, 2005). It is difficult to generalize the physical demands of refereeing during matches but findings suggest the importance of individualizing the evaluation of fatigue in referees (Weston et al., 2011a; Costa et al., 2013).

In addition to the physical demands, a top referee faces high psychological demands, making around 137 observable decisions per match (Helsen and Bultynck, 2004). If we also add non-observable decisions and take into account an average effective playing time of 51 min (Miyamura et al., 1997) it is suggested that a top-class referee makes 3–4 decisions per minute (Helsen and Bultynck, 2004). Among these decisions 28% are about fouls and misconduct which can impact match result or players’ health (Fuller et al., 2004). Van Meerbeek et al. (1987) counted the number of correct and incorrect decisions related to specific laws of the game during the 1986 World Cup in Mexico. They found that of all the decisions made in 16 games, 17% were incorrect (range: 11–35%). Similarly, other studies have looked at decisions made during football games in comparison to expert panel decisions via post-match analysis and found disagreement in 21–40% of the decisions (Fuller et al., 2004; Mascarenhas et al., 2009). This difference could be related to several factors that influence the referee’s decisions in real environments, for example: prior decisions; team reputation; crowd noise; home advantage and; players’ physical stature (Plessner and Betsch, 2001; Jones et al., 2002; Nevill et al., 2002; Van Quaquebeke and Giessner, 2010).

Van Meerbeek et al. (1987) concluded that the number of observed decisions is uniformly distributed throughout a match. This means that referees are equally focused from the beginning of a match to its end independently of their level of fatigue. In a naturalistic study, Mascarenhas et al. (2009) investigated the combined effect of exercise and physiological factors on DM. They concluded that referees make on average 64% accurate decisions, and accuracy levels were not related to movement speed, heart rate, or cumulative distance covered. The authors concluded that none of these variables individually predicts decision accuracy but rather a more complex, multivariate relationship between them (Mascarenhas et al., 2009). Therefore, on the field, DM might be influenced by the specific context of the match and the access to the most accurate information. Throughout their own training and development, referees become experts in using this perceptual information (de Oliveira et al., 2014). Importantly, referees need to move with the game play in order to make that information available. Several authors argue for a bidirectional link between actions and perception as a base for accurate DM in different contexts (e.g., Newell, 1986; de Oliveira et al., 2009, 2014). This is important because under strenuous physical and physiological conditions referees are less able to position themselves in the play situation which may result in visual perception and hence DM being negatively affected. Indeed, Mallo et al. (2012) investigated the accuracy of referees’ decisions during the FIFA Confederation Cup 2009 and found that incorrect decisions occurred twice as often in the second half of the match than in the first half suggesting the influence of fatigue.

It is also known that the timing of visual perception is crucial in expert anticipation, actions and decisions (Oudejans et al., 2000; de Oliveira et al., 2008), but only three studies have investigated gaze behaviors in referees (Bard et al., 1980; Catteeuw et al., 2009; Hancock and Ste-Marie, 2013). Investigating the gaze behavior of referees would help to understand what information sources referees attend to, the timing for using information, and the potential role of distractions. In a game situation, referees have plenty of distractions (e.g., Lex et al., 2014) and have to attend and react to new information appearing while maintaining concentration over 90 min (Pietraszewski et al., 2014).

Bard et al. (1980) found significant differences between elite and novice gymnastic referees when they measured gaze behavior and accuracy. In ice hockey, expert referees were better than lower-level referees in DM, but did not differ in gaze behavior (Hancock and Ste-Marie, 2013). In football, players’ gaze behaviors toward relevant open spaces; more fixations of shorter duration have been associated with better decisions (Mann et al., 2009). The only study investigating gaze behavior in football referees, however, found no differences in scan patterns between international and national assistant referees during match play (Catteeuw et al., 2009). The contrasting results of these studies regarding visual search patterns and their relationship with accurate DM suggest that further research is required to understand referees’ visual perception in relation to DM. Such research should focus on the interactions between environment, movement patterns, performance level, gaze behavior and DM.

Heat exposure negatively impacts both physiological and cognitive performance (Racinais et al., 2008). During passive hyperthermia, decrements in memory and sustained attention were observed with an increase in core temperature (Gaoua et al., 2011b). The variation in skin temperature also produced decrements in complex cognitive tasks (Gaoua et al., 2011a, 2012). Performance of a rapid visual processing task showed an increase in rapid inaccurate responses in a hot environment compared to a thermo-neutral environment, suggesting an increase in impulsivity (Gaoua et al., 2011b). Dickman and Meyer (1988) proposed that increases in impulsivity alter cognitive performance at the decision stage (Exposito and Pueyo, 1997, but see Cisek and Kalaska, 2010). DM can also be adversely affected during exercise in a hot environment (Ernwein and Keller, 1998) and reductions in both working memory capacity and in the ability to analyze and retain visual information have been observed when core temperature is increased to 38.5°C through exercise (Hocking et al., 2001).

In the other extreme, cold environmental temperatures can also significantly affect cognitive performance (Palinkas, 2001; Pilcher et al., 2002) and particularly concentration, vigilance, memory, and reasoning (Taylor et al., 2016). Specifically, moderate cooling leads to decreases in simple cognitive tasks (Palinkas, 2001) while more severe cold exposure (-20 to 10°C) decreases memory performance (Patil et al., 1995), vigilance (Flouris et al., 2007), and DM (Watkins et al., 2014). Cold also decreases the intensity of perceptual responses (Acevedo and Ekkekakis, 2001) possibly compromising DM performance. Most of the research investigating the effect of cold comes from work settings where, as temperatures decrease, the frequency of errors increases. Researchers demonstrated that the number of errors significantly increased in ambient temperatures of 5°C compared to 22.5°C (Pilcher et al., 2002). More recently, responses to cold were investigated and showed a decrease in cognitive performance in cold in a control population but not in elite alpine skiers. Authors suggested that the usual training in cold environments of the skiers made it possible for them to maintain attention on the task. This occurred at the expense of a longer duration to find the correct answer (Racinais et al., 2016). However, referees are not always habituated to cold and are required to make fast and accurate decisions.

Carling et al. (2011) investigated the physical activity profiles of professional soccer players in official matches played in a cold environment and demonstrated that physical performance did not decrease in cold environment, although HIR only reached its optimum toward the end of the first half. In hot environments players seemed to modify their game by reducing the distances covered during matches (Racinais et al., 2012). In another study Mohr et al. (2010) examined fatigue in elite soccer played in hot conditions and concluded that heat reduces HIR toward the end of the match. Given that the referee has to stay close to the game play, these results suggest that extreme conditions may compromise referees’ ability to maintain proximity with key events during the match, therefore possibly impacting on the information available for the referee to perceive and act/judge accurately (Dicks et al., 2010; Orth et al., 2014). Despite evidence that heat compromises both physical performance and DM (Mohr et al., 2010; Gaoua et al., 2011b; Racinais et al., 2012) only a few studies investigated its specific effect on referees’ DM.

In their recent study, Taylor et al. (2014) investigated the effect of hot and cold exposure on DM after a 90-min intermittent treadmill protocol simulating match-play. In this experiment exposure to hot or cold environments did not affect the DM ability of football referees. However, these results may be limited because there was no significant difference in referees’ core temperatures between the cold and hot conditions (8 and 30°C, respectively). This means that, in the hot condition, the recorded core temperatures (<38.5°C) were substantially below those observed in players during football matches in the heat (≈40°C, Racinais et al., 2012). The authors acknowledged that their protocol may not have elicited the strain experienced by referees during a real football match in the heat. In another study, goal line officials’ performance only decreased when exposed to a cold environment. Using a simple regression analysis, the authors suggested that cognitive performances improved at computerized tasks with the increase in skin temperature and decreased with the decrease in skin temperature (Watkins et al., 2014). These results contradict previous findings that suggest decrements in cognitive performance with the increase in skin temperature in hot environments (Gaoua et al., 2012). In addition, a recent study showed that a decrease in skin temperature during passive cold exposure had no impact on cognitive performance in elite skiers but compromised a control group’s performance (Racinais et al., 2016) possibly because skiers were habituated to cold. These studies clearly highlight the need of further research to identify the effect of both cold and hot environments on referees’ decisions in natural match environments.