Serotonin is present in the brain. It is considered to have mood, happiness, and anxiety regulating properties. Serotonin deficiency has been related to depression, whilst greater amounts of the hormone may reduce alertness. Stress can increase serotonin levels in the brain. The answer is stress. When we are under stress our body produces more of the hormone cortisol which helps us deal with that stress but if you keep your body under constant stress it will always produce more cortisol and this can lead to depression. Cortisol also causes many other problems beyond just making us depressed, it can cause weight gain, diabetes, heart disease, and even increased risk of cancer. So reducing its effect on our body by lowering cortisol levels would be a good thing. The main source of cortisol is from your adrenal glands. If you look at a diagram of an adrenal gland, it looks like a little ball of flesh sitting on top of each kidney. The hormones produced by this tissue control many important functions in your body including blood pressure, heartbeat, sugar level, and immune system response. Dysfunction of these glands can lead to chronic fatigue, pain, depression, memory problems, difficulty concentrating, irritability, sleep disorders, and weight gain.