Stress causes the adrenal glands to start producing cortisol, among other hormones. The purpose of cortisol is to make more glucose, a sugar, available to the brain while causing the immune system to limit its functioning. The adrenal glands also release adrenaline, which increases one's heart rate and prepares them to take action. The body usually returns to normal after a moment of stress, if the body uses up the cortisol available to it. However, if one is continually in a state of stress and the cortisol is not used, one's bodily functions are interrupted and long-term health problems can result, including heart disease and high blood pressure and reduced immunity from disease. In addition, prolonged stress can result in weight gain, decreased memory, and problems with mental health, including depression, anxiety, and other issues.
King David is a character in the Bible who suffered with what we might recognize with depression and insomnia, particularly after his son Absalom rebelled against him.