Reduced productivity
As Forbes states, the number one typical sign of a looming burnout is reduced productivity. A salesperson who used to be one of the sales aces is suddenly struggling to achieve even the lowest of goals. A developer who used to write hundreds of lines of code a day suddenly has problems with even a simple assignment. So as soon as you see a subordinate's productivity drop, beware.
Isolation from the rest of the team
An employee who is overworked and at risk of burnout is not usually in the mood to socialise much. So they cease to be interested in their colleagues, isolate themselves and do not want to talk much with anyone. They avoid get-togethers, excuse themselves from meetings and do not mention their personal life. Thus even a once talkative person becomes silent.
Physical symptoms
Although burnout is primarily a psychological condition, it is usually accompanied by psychosomatic symptoms. These include increased sickness, abdominal pain, headaches, insomnia and subsequent daytime fatigue. Therefore, if a subordinate is experiencing reduced productivity, isolation from others and increased sickness, you should definitely be alert and not take the situation lightly.
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