The level of employee engagement varies on a spectrum from the state of disengagement to the state of burnout. The sweet spot – high and healthy level of engagement, is found somewhere in between these two extremes.
Macey and his colleagues distinguish two causes for employees not experiencing engagement:
disengagement due to lack of support for engagement – lack of trust, fairness, challenging and meaningful work;
not being engaged as a result of too much support for engagement – the person becomes so engaged that it leads to exhaustion and burnout.
The relationship between engaging work conditions, work engagement, and burnout is illustrated in the figure below (based on Warr).
Research and consulting firm Gallup distinguish two kinds of disengaged employees:
not engaged employees, who are only little or not at all concerned about their customers, the productivity of their work, or mission of their team and organization. They do only as much as is required to keep their jobs. These employees are essentially checked out.
actively disengaged employees, who are not only disengaged and unhappy; they spend most of their time acting out their unhappiness and diminishing the accomplishments by other – engaged colleagues.
Disengagement might be avoided by focusing on what creates engagement; however, even with all the support, the right situational characteristics, the ideal circumstances, and the best of leadership, some people still may not be engaged.
Based on more than 30 years long research, Maslach, Schaufeli, and Leiter place burnout at the opposite end of the engagement continuum.
Job burnout refers to the response of chronic emotional and interpersonal stressors on the job and is defined by three dimensions – exhaustion, cynicism and sense of inefficacy.
Maslach, Schaufeli, and Leiter define dimensions of burnout as follows:
Exhaustion – the basic individual dimension of burnout, which refers to being depleted of one’s own physical and emotional resources;
Cynicism – the interpersonal dimension of burnout, which is related to excessive detachment from various aspects of the job;
Based on the above-described theories, both – lack of support as well as too much support for engagement lead to disengagement or burnout. Engagement happens only when there is just the right amount of support – not too little and not too much. Job Demands-Resources model (JD-R) by Bakker and Demerouti
Burnout results when one's energy turns into exhaustion, involvement becomes cynicism, and efficacy becomes inefficacy. 
Based on the above-described theories, both – lack of support as well as too much support for engagement lead to disengagement or burnout. Engagement happens only when there is just the right amount of support – not too little and not too much. Job Demands-Resources model (JD-R) by Bakker and Demerouti illustrates an approach to finding the right balance.
The model suggests that there are two types of job components affecting employees' engagement levels: job demands and job resources.
Demands in this context are those organizational, social, and psychological aspects of job that require sustained physical and emotional effort and are associated with certain psychological and physiological costs.
Job demands are not necessarily negative, however, they can turn into stressors, when meeting such demands requires high effort, from which the employee does not recover properly.
Resources are those physical, psychological, social, or organizational aspects of the job, that decrease job demands, foster employee’s ability to work towards the work goals, and also stimulate personal growth and development.
Therefore, resources are not only necessary to deal with the demands, but also in their own right.
Job demands and resources are negatively related, as high demands might affect the allocation of job resources, while high levels of job resources may reduce the demands.
In conclusion, too much resources in combination with too little demands lead to disengagement, since employees don’t feel challenged enough to fully engage and invest their energy while dealing with such low-demand circumstances. At the same time, too much demands in combination with too little resources lead to burnout, since employees do not have enough resources to deal with the overly challenging demands. Therefore, the key to engagement and peak performance is the right balance between demands and resources, where demands are challenging and resources sufficient.
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