Light And The Human Endocrine System

Light affects our bodies in two ways. In the first, light impinges on the retina of our eyes and, through our vision system, affects our metabolism and our endocrine and hormone systems. In the second, it interacts with our skin by way of photosynthesis and produces vitamin D.

Most of our body's life-sustaining functions are controlled by the hypothalamus, an area of the brain below the thalamus (Figure 3.1). The hypothalamus is responsible for a number of metabolic processes and for such autonomic activities as energy and fluid balance, growth and maturation, circulation, breathing, emotional balance, reproduction, heat regulation, and the circadian cycle. It links the nervous system to the endocrine system by synthesizing and secreting neurohormones as needed; these in turn control the secretion of hormones from the anterior pituitary gland (Figure 3.2).

Our daily activity and sleep rhythms are regulated by a control center in the hypothalamus called the suprachais-matic nucleus (SCN), also known as the body clock. The body clock needs to receive signals to tell it when to shut down and prepare for sleep and when to produce the active waking

Figure 3.1 The hypothalamus part of the brain.
Figure 3.2 The pineal and pituitary glands in the brain.

hormones. It generally requires daily resetting by external time cues. This process is called entrainment. The day-night cycle of a 24-hour solar day is the main environmental signal entraining the clock and the rhythms driven by it. Our body must receive these cues with the right amount of light at the right time and frequency. When it doesn't, the internal clock is disturbed, and so are many of our bodily functions.

The most powerful signal is bright light, such as sunshine (Moore and Eichler, 1972; Stephan and Zucker,1972; Inouye and Kawamura, 1979). The SCN serves as an hourglass timer or a pacemaker for our internal clock or circadian rhythm and is directly influenced by light intensity. Daylight serves as a catalyst for the secretion of hormones from the pineal gland (Figure 3.2 ) , namely serotonin and melatonin. The level of melatonin determines the energy and activity levels in our bodies. At darkness or low light levels, melatonin secretion increases and drowsiness occurs. Daylight suppresses the production of melatonin and fosters an alert state of mind by secreting serotonin. People who live and work in window-less environments or in places lacking adequate light may be at risk of having their internal clock continually disturbed.

Age (years)

Figure 3.3 Variation of melatonin secretion with age.

Age (years)

Figure 3.3 Variation of melatonin secretion with age.

Melatonin, the so-called 'natural nightcap,' directly acts on the SCN to influence circadian rhythms (Weaver et al., 1993). At night melatonin is produced to help our bodies regulate our light-dark diurnal cycles. Melatonin is an important antioxidant and can neutralize some agents (the hydroxyl radical agents) that damage cells and DNA that are believed to be a contributing cause of some internal cancers (Reiter, 1995).

Serotonin, a hormone first discovered in 1933, is the neurotransmitter identified in many psychiatric disorders including depression, anorexia, bulimia, obsessive-compulsive disorder, and social anxiety. Serotonin is an important regulator of these disorders. If we compare the brain to a car engine, serotonin can be considered the 'oil' of the brain. During long periods of high stress, serotonin is used up, exceeding its replenishment rate. Prolonged stress lowers serotonin levels in the brain and a stress-induced depression may take place. The less serotonin available in the brain, the more severe is the depression and related symptoms. Low serotonin function is thought to result in a type of depression characterized by such symptoms as suicidal thoughts and feelings of sadness, worthlessness, and guilt.

The actions of serotonin and melatonin on our circadian rhythm function in opposition, with serotonin stimulating us during the daytime and melatonin inducing sleep at night; however, they should be seen as complementary to each other and equally essential to our circadian rhythm. Healthy young and middle-aged adults usually secrete about 5 to 25 micrograms of melatonin each night. The levels peak at ages 2 to 5 years and then progressively decline by 10-15% per decade (Figure 3.3) (Brown et al., 1979; Waldhauser et al., 1984; Zhdanova et al., 1998; Grivas and Savvidou, 2007).

Scientists speculate that this decline may explain why young people have fewer problems sleeping than older people.

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