Cortisol is an adrenal hormone that is released during physical and psychological stress and plays an important role in digestion, the sleep-wake rhythm and the immune system. Cortisol also shows a clear day/night rhythm, with a peak during getting up. Sufficient cortisol is vital. People with Addison’s disease make far too little cortisol. If they do not take corticosteroids, they can become unconscious and even die. People with Cushing’s disease make too much cortisol.