Biological Night (IMAGE) NIH/National Heart, Lung and Blood Institute Caption 'Biological night' is defined as the period between the onset and cessation of melatonin secretion. During this period, melatonin is secreted, blood cortisol levels rise, core body temperature goes down, and we become sleepy. Melatonin is produced only during darkness and stops upon optic exposure to bright light, with light in the blue portion of the visible spectrum proving the most potent at suppressing production. Credit Angela Spivey, 10.1289/ehp.118-a28 Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.