Our bodies rely on the information received through the stimuli produce due to the presence of light. Fundamentally light controls our bodily clock or circadian rhythms, telling us when to wake and when to sleep. Through the presence of light and the neural stimulation of the pineal gland in the brain, the hormones Melatonin & Cortisol are secreted into the bloodstream, which in turn help to control our bodily or circadian rhythms.
Cortisol produced by the adrenal gland is released in response to stress. Its primary functions are:
- increase blood sugar through gluconeogenesis
- suppress the immune system
- aid in fat, protein and carbohydrate metabolism.
Circulating levels of the hormone melatonin vary in a daily cycle, thereby allowing the entrainment of the circadian rhythms of several biological functions. Melatonin is secreted into the blood by the pineal gland in the brain. Known as the "hormone of darkness", it is secreted in darkness in both day-active (diurnal) and night-active (nocturnal) animals.
Production of melatonin by the pineal gland is inhibited by light and permitted by darkness. For this reason melatonin has been called "the hormone of darkness". Its onset each evening is called the Dim-Light Melatonin Onset (DLMO). Secretion of melatonin as well as its level in the blood, peaks in the middle of the night, and gradually falls during the second half of the night, with normal variations in timing, according to an individual's chronotype.
It is principally blue light, around 460 to 480nm, which suppresses melatonin increasingly with increased light intensity and length of exposure - melatonin promotes sleepiness.
Prolonged exposure to abnormal lighting conditions, e.g. working under ground, can disrupt our 24-hour cycle. Long term deprivation of light can lead to Seasonally Affective Disorder (SAD), which can leave people feeling low, tired and even depressed; these symptoms are common in Northern European countries where the winter days are very short, with low levels of light, or through working night shifts or within confined environments e.g. underground.