Caffeine is a stimulant that directly acts on your central nervous system. It stimulates the brain to release cortisol hormones. When our body is under stress, the brain sends signal to secrete cortisol hormones for dealing with stress. Caffeine overdose exerts pressure on your adrenal glands for repeated secretion of anti-stress hormones thus overburdening them. Ultimately, your adrenal glands can no longer secrete these hormones thus reducing the body’s stress handling ability. Also, increased cortisol levels impede sleep hormone secretion causing insomnia resulting in chronic tiredness.