So we just talked about testosterone, but I also said that human growth hormone was also an important hormone. Growth hormone is released from the anterior pituitary gland and secreted into the bloodstream. Its production and release is controlled by the hypothalamus like the other hormones. Now, somatostatin that releases the hypothalamus inhibits the release of growth hormone from the anterior pituitary and you will see the significance of this shortly. At rest somatostatin is inhibiting growth hormone. Now, secretion of growth hormone is slightly higher in women than men with the highest levels observed at puberty. And secretion decreases with age by about 14% per decade. And this is why you see these artificial growth hormone supplements available as people age. Growth hormone is secreted in 6 to 12 discrete pulses per day. And the largest pulse comes around one hour after the onset of night time sleep. And sleep is a powerful stimulant for growth hormone release. Another powerful stimulus is exercise, and here I've added a bout of strenuous exercise into this graph. Growth hormone has a variety of roles, including the growth of a muscle, it also works to strengthen bone and the collagen which is in the ligaments and tendons and it's also involved in fat metabolism. Now growth hormone does not appear to affect tissue directly. It stimulates insulin like growth factor into action and under normal conditions when there's sufficient insulin like growth factor available, it stimulates the production of somatostatin that blocks the release of growth hormone from the pituitary and this is what I told you was significant about somatostatin. It sees a lot of IGF available, then there is the body doesn't think it needs any growth hormone to be released so it's going to block it. And it's thought that exercise blocks this inhibitory feedback pathway, and this is why growth hormone spikes during exercise.