The level of circulating melatonin can be detected in samples of blood and saliva, and this is used in clinical research to identify internal circadian rhythms. In humans, nocturnal levels of melatonin decrease across puberty. The importance of pineal melatonin in human biology is not clear, although it may help to synchronise circadian rhythms in different parts of the body. reproduction, behaviour and coat growth) in response to changing day length. In many animals (including a wide range of mammals and birds), melatonin from the pineal gland is essential for the regulation of the body’s seasonal biology (e.g. In addition to its circadian rhythm, melatonin levels also have a seasonal (or circannual) rhythm, with higher levels in the autumn and winter when nights are longer, and lower levels in the spring and summer. Night-time levels of melatonin are at least 10-fold higher than daytime concentrations. Tissues expressing proteins called receptors specific for melatonin are able to detect the peak in circulating melatonin at night and this signals to the body that it is night-time. Melatonin is carried by the circulatory system from the brain to all areas of the body. Once produced, it is secreted into the blood stream and cerebrospinal fluid (the fluid around the brain & spinal cord) and conveys signals to distant organs. The production and release of melatonin from the pineal gland occurs with a clear daily (circadian) rhythm, with peak levels occurring at night. Night-time is detected by reduced light entering the eyes (left), and the arrow shows the melatonin secretion signal sent by the optic nerve to the pineal gland once darkness has fallen.

Melatonin (blue) is produced naturally from the amino acid tryptophan, by the pineal gland (purple) at night-time. Melatonin is produced by various tissues in the body, although the major source is the pineal gland in the brain.

Epinephrine is produced by which gland