During the phase of selye's theory of stress, considerable energy is expended to adapt to the stressor.

According to Selye's theory of stress, during the condition of the stress, the body undergoes the general adaptation syndrome, which occurs in three stages. These three stages are fight or flight, resistance reaction, and exhaustion.

The flight or fight reaction is stimulated by the hypothalamus. It results in the excitement of the sympathetic division of the autonomic nervous system. The resistance reaction is caused by the hormones released by the hypothalamus; this is long lasting and provides ATP (adenosine triphosphate) molecules for the counter reaction to the stress.  

As the result of use of ATP and other energy resource from body during the resistance reaction, the body becomes deprived of energy. This causes the exhaustion in the body.  

So, the resistance phase of the stress expands a considerable amount of energy, which causes exhaustion.  

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