Rabiu M.I, et al. Bayero J Biomed Sci 2020; 5(1): 617-623 Date submitted-04/06/2020 Date Accepted-05/08/2020 617 Effect of Light at Night on Blood Level of Luteinizing Hormone, Follicle Stimulating Hormone and Testosterone in Male Wistar Rats *Rabiu M.I., Dissi G.M., Salim M.A.& Salisu A.I., Department of Human Physiology, Faculty of Basic Medical Sciences Bayero University Kano, Nigeria *Corresponding Author:Rabiu M.I. Corresponding E-mail: rabiumusaisah@gmail.com. Abstract Excessive exposure to artificial light or dark environments causes the disruption in the circadian rhythm of the day- night cycle which subsequently results in various diseases. The aim of this study was to investigate the effect of light at night on Luteinizing Hormone (LH), Follicle Stimulating Hormone (FSH) and Testosterone in male Wistar rats. Thirty-five adult male Wistar rats (7-8 weeks old) mean weights of 110grams were randomly assigned into five groups of seven rats: Group A (control) was maintained under 12:12 light/dark cycle (12 hours light exposure). Group B was maintained under 18:6 light/dark cycle (18 hours light exposure). Group C was maintained under 6:18 light/dark cycle (6 hours light exposure). Group D was maintained under 0:24 light/dark cycle (24 hours dark exposure). Group E was maintained under 24:0 light/dark cycle (24 hours light exposure). The study last for three weeks. The rats were anaesthetized using chloroform by inhalation. The blood sample was collected by cardiac puncture. LH, FSH and testosterone were evaluated using ELISA kits according to the manufacturers’ guidelines. The results showed that exposure to excessive artificial light or dark environments disrupted circadian rhythm. There were no statistically significant differences in weight gain among groups (p=0.805). There were significant increase in the mean values of LH and Testosterone in rats exposed to 24 hours light compared to the control; 64.45±1.59mIU/ml versus 51.63±3.55mIU/ml (p=0.011) and 1.89±0.08mIU/ml versus 1.28±0.06mIU/ml (p=0.001) respectively. However, there was no significant difference in the levels of FSH between study and control groups (p=0.565). This study demonstrates that exposure to excessive artificial light or dark environments disrupts circadian rhythm; exposure to artificial light at night significantly increases the concentration of LH and Testosterone but no effect on the concentration of FSH in male Wister rats. Keywords: Circadian Rhythm, Luteinizing Hormone, Follicle Stimulating Hormone, Testosterone, Introduction One of the most rapid and radical changes that began at the end of the nineteenth century is the substitution of the natural 24 hours light/dark cycle for around-the- clock artificial light that differs markedly in the spectrum, intensity, and temporal patterning. [1] In mammals, environmental light is the main input that synchronizes the master circadian clock in the hypothalamic suprachiasmatic nucleus (SCN) to the outside world. The SCN communicates the external time of day to other brain areas, including areas that regulate sleep and food intake such as the ventrolateral preoptic nucleus and the lateral hypothalamus. [2] The SCN also communicates with peripheral clocks in metabolic tissues such as the liver and adipose tissue and regulates glucose metabolism. [3,4] Observational studies in humans shows that exposure to bedroom light at night was associated with insomnia, obesity, and dyslipidemia. [5] In nocturnal rodents, the bright light at night increased sleep and decreased food intake. [6] Long term exposure to constant light disrupted circadian rhythmic behaviour, decreased total time spent awake and caused obesity and type 2 diabetes. [7] Circadian rhythm in human being and other mammals like rats can be assessed by measuring the circadian rhythmic profile in serum level of melatonin, cortisol, or core body temperature. In addition, rating scales such as the Morningness-Eveningness Questionnaire (MEQ) or Composite Scale also are useful to assess the sleep/wake cycle or circadian patterns of human. [8] Light at Night (LAN), acting through endocrine disruption, have been implicated in an increased risk of metabolic diseases including breast cancer. [9] A series of studies have shown that light-dark patterns incident on the retina set the timing of the master clock. [10] Circadian disruption resulting from chronic exposure to irregular light-dark patterns plays a role in diseases such as cardiovascular disease, diabetes, obesity, and tumour growth. [11,12,13] One population at greater risk for exposure to LAN and irregular light-dark patterns