Do Photoperiodic Changes in Melatonin Secretion Detrimentally Affect the Female Reproductive Cycle ?

Melatonin, better known as "the hormone of darkness," is secreted by the pineal gland during the night and helps us fall asleep. Because its internal regulation depends on light, melatonin is part of chronobiology, the study of biological mechanisms and their adaptations to lunar and solar related rhythms (Klein et al. 1991). Therefore, photoperiod changes greatly impact melatonin concentration, influencing changes in neuronal and hormonal mechanisms of the photo neuro-endocrine systems, namely reproduction. Research has shown that a disruption in the circadian rhythm of melatonin due to photoperiod changes detrimentally affects the rhythmic function of the female reproductive cycle. Research from journals, articles, and printed books has shown that both exogenous and endogenous features contribute to the reproductive cycle and that the internal mechanisms are entrained by environmental cues. Photoresponsiveness of the reproductive system is mediated by the internal biological clock, transcriptional factors, period genes, photic input, GnRH (gonadotropin-releasing hormone) neurons, and melatonin secretions. Specifically, melatonin secretions directly affect reproductive function either through stimulatory or inhibitory pathways. Seasonally breeding animals interpret photic information according to melatonin secretions. This clearly points to a relationship of melatonin with photic and reproductive qualities. Research is ongoing with various species of animals in hope of uncovering the mystery of the connection between light, melatonin, and reproduction, which may be helpful in understanding the effects of photoneuroendocrinology on the female human. INTRODUCTION In the industrial world of the 21st century, work at night seems to be unavoidable. Erhard Haus claims that 17% of the working world is involved in some kind of rotating shift work, permanent night shift, or trans-meridian travel (Haus and Smolensky 2006). Nurses work rotating shifts, wholesale food producers drive through the night, bakers bake in the wee hours of the morning, medical school students intern around the clock, security guards work night duty, and traveling businessmen fly many miles a month. What they all have in common is that they are on arrhythmic sleep schedules. The diabetic frequent flyer knows to keep insulin medication handy because glucose levels can fluctuate in response to confusing messages of night and day the body receives when traveling across several time zones, for traveling eastward shortens the day while traveling westward lengthens the day (Keystone and Kozarsky 2011). The body runs on an internal rhythmic schedule and may not adapt easily to extreme changes in the timing of surrounding environments. The World Health Organization has blamed shift work for the increasing occurrences of cardiac, gastrointestinal, and reproductive disorders and certain cancers (Longo et al. 2012). Melatonin is known to inhibit estrogen dependent disorders, including breast cancer. If sleep schedules are compromised, melatonin may not be able to guard the body from the invasion of cancer. Epidemiologic studies on women show a correlation between shift work employment and the incidence of breast and colon cancers. Melatonin disturbances have shown to affect proper reproductive function as well. Flight attendants and shift work employees, two groups of people who suffer from disrupted day/night schedules and 61