How Does Light Affect the Human Functions?

Human beings are the product of habits and heritage. Before the advent of alarm clocks, many farmers woke up hearing the rooster crowing, announcing the arrival of morning. They milked their cows, worked in the farm and went into bed at night. There was no electricity. So, daylight announced the initiation and termination of many activities.

Modern life style differs significantly from these early days. Most of us wake up in the morning, not by hearing a rooster crow or by feeling the golden rays of sunlight slowly drifting into our rooms; we wake up by the alarm clock or by the clock radio. Many of us have tough time getting up at the first time; so we set the "snooze" button to give us a little more of precious time to sleep. The windows have heavy drapes, so most of us do not see the sunlight except when we peek outside. In the evening, many of us stay awake to watch the late night shows. (Now we have light night shows and late late night shows to keep us company till the wee hours of the night.)

The problem is that our system needs time to sleep. Studies on animals have shown that they have definite patterns they follow every day depending on the season. In autumn, most of the plants and animals get ready to go into "hibernation" for the winter period. Many birds migrate to south for the winter. During this period, they do not eat much (There is not much food to be found.) But, come spring, nature become very lively. The birds return from the south. The trees starts the new growth.

Many animals are found to time the events in their lives depending on the season, so that the functions can be accomplished at the most effective way. For example, lambs are born only in the spring when there is plentiful of food for the mother to nurse the newborn. Most of the animal species coordinate the mating time so that the birth occurs in the season when there is plenty of food available. In the tropical rainforests, birds wait till the dry season to breed. In Arctic, the breeding is timed to coincide with the melting of snow and ice.

The question is how do animals know how to predict the seasons in advance? Is it the temperature fluctuations? It cannot be, because, sometimes we have the "so called Indian Summer" in fall; but the birds do fine. It turns out that the most important factor is the day light; or more specifically the day/night cycle. Animals and plants sense the shortening of the days in the fall and perceive the arrival of winter. In spring, the lengthening of the day signifies the arrival of spring and summer. Most of the expert horticulturists know about this. They manipulate the "day light hours" (or photoperiod) to coax the poinsettia to bloom in time for the Christmas season, daylilies to bloom for the Easter (although Easter can be in March or April), etc.

It turns out that human beings are also influenced by the light. Light determines our sleep/wake cycle. In most animals and humans, the desire to sleep is brought on by secretion of a hormone called melatonin. Melatonin is produced in a tiny gland known as the pineal gland. In the evening the pineal gland reacts to the diminishing levels of daylight and starts to produce melatonin, which is then released into the blood and flows through the body making us drowsy. Its secretion peaks in the middle of the night during our heaviest hours of sleep. In the morning, bright light shining through the eye reaches the pineal gland which reacts by switching off the production of melatonin, thus removing the desire to sleep.

The pineal gland is linked up to the rest of the hormonal system. Consequently melatonin production also influences the functioning of other parts of the body. During darkness and sleep, melatonin modifies the secretion of hormones from organs such as the pituitary, the master gland of the hormonal system. The pituitary in turn regulates the secretion of hormones controlling growth, milk production, egg and sperm production. It also regulates the action of the thyroid gland, which is concerned with metabolism, and the adrenal glands, which control excretion of the body's waste. It is obvious then that fluctuations in light and darkness according to the seasons of the year will influence rhythms of growth, reproduction and activity in animals and indeed humans.

Statistics show that despite living and working in "closed structures", our bodies still respond to the external environment and to its seasonal variability in duration and intensity. Scientist have found that growth rates in children are affected by the seasons. For example, surveys carried out in Germany, Sweden and Scotland show that height and weight increase is more predominant in the spring and early summer. In many countries the rate of conception peaks in the summer when the hours of daylight are longest. In numerous trials the seasons have been seen to influence the timing and duration of sleep, pain threshold, alertness, eating habits, mood, the onset of menstruation in women and sexual activity.

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