Stress has been associated with the activation of the hypothalamic-pituitary-adrenal (HPA) and the sympatho-adrenal-medullary axes, whereas corticotropin-releasing hormone (CRH) and cortisol (products of the hypothalamus and adrenals, respectively), and catecholamines (products of the sympathetic system) are known to cause arousal and sleeplessness to humans and animals. On the other hand, sleep and particularly deep sleep appears to have an “anti-stress” effect as it is associated with an inhibitory effect on the stress system including its main two components, the HPA axis and the sympathetic system.