Raheem Sterling has been criticised by some for asking not to feature in England’s starting XI for the Euro 2016 qualifier against Estonia because he was “a little tired”. However, the Liverpool forward could be experiencing the early symptoms of burnout after a hectic year.

Sterling has played 1,682 minutes of football since the end of May. In 2013, through the same period, the 19-year-old played 615 minutes. He was one of Liverpool’s star performers during their title challenge last season and was regarded as England’s big hope for their ill-fated World Cup campaign in Brazil.

The majority, including Roy Hodgson, have supported and understood Sterling’s stance, although others have questioned his judgment. Alan Shearer, the former England striker, wrote in the Sun: “The working man who is up at 6am and home at 8pm does not want to hear about how tired a 19-year-old professional footballer is. I genuinely have never heard something like that in my career. Certainly not in a squad I was involved in.”

According to experts, burnout has as much to do with an elite athlete’s psychological wellbeing as his physical state. The two are inextricably linked but mental fatigue can have significant knock-on effects to an athlete’s performance.

Full-blown burnout can be prevented even if early symptoms are prevalent, as could be the case with Sterling. It is much more common in youngsters than experienced professionals, who have become accustomed to dealing with the pressures and expectations of elite sport.

Dr Andrew Hill, a leading academic, argues that external expectation is one of the biggest causes. Given the levels that Sterling reached for Liverpool last season, and the fact that he is regarded as one of the most talented attacking options in the country, such pressure could be a cause of his fatigue.

Dr Hill told the Guardian: “We understand it to be much more psychological than physiological. There is often confusion when people talk about overtraining, too many games or fixture congestions; what they are talking about is physiological fatigue but burnout is normally about psychological exhaustion. They are correlated. For every match he plays there is going to be a psychological expenditure, associated with preparing for games and competing in games.”

What are the symptoms?

There are three main signs of burnout. The first is emotional and physical exhaustion, not restricted to training and matches but with crossovers into everyday life. Often there is a reduced sense of accomplishment, with individuals perceiving that they are not achieving what they should and they are falling short. The third symptom is devaluation, when athletes become more cynical and have an increasingly negative attitude towards the sport.

What are the causes?

“People who burn out are not those who perceive that everything is going really well,” Dr Hill says. “It happens when things are difficult or you are not getting the success that you want. That is when it manifests itself.” A sense of external pressure can potentially be a catalyst for burnout. The perception of expectations and demands increasing can be important; once an athlete thinks his coping mechanisms are not adequate then stress can begin to accrue.

Are youngsters more susceptible?

Research shows that burnout is more prevalent in younger athletes. More experienced elite athletes have probably developed the coping mechanisms necessary for a career at the top level, although this does not mean that burnout will not happen later on. More commonly, though, fatigue occurs in an athlete’s formative years, when he is still learning to cope with the psychological and physiological pressures. At that time in his career there is still a question mark over whether he is going to reach his mid-20s having learned to deal with such demands.

What are the solutions and how long can it last?

Athletes can experience the early signs of burnout without having the full, longer-term symptoms. Dr Hill says: “If someone is still able to function and play it is much more likely to be signs of the early onset of burnout, which can be prevented with rest and developing the psychological skills that make you able to cope with stress.”

Given that Sterling played for 26 minutes against Estonia and had a decisive impact, this could be the case. He has played in 14 competitive matches since mid-August and it will be up to Liverpool, Brendan Rodgers and Hodgson to decide how much rest he requires.