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Stress and fatigue reduce testosterone just as much as age does
Age is an important factor that determines the amount of testosterone circulating in men's bodies, but it's not the only one. American army researchers discovered that stress and fatigue may be just as important factors. It could be the case that men's testosterone levels rise if they reduce their stress levels and make sure they are well rested.
The fatigue that the soldiers reported also played a role. The higher the soldiers scored their tiredness, the lower their testosterone level.
The same was true for stress. The more stress the men experienced, the lower their testosterone level.
High scores for stress and fatigue had a similar correlation with low testosterone levels as age did. This study revealed that BMI and sleep did not play a significant role.
The figure below shows the correlation between testosterone, stress, fatigue, age, BMI and sleep. The closer a correlation is to 1 or -1, the stronger the relationship.