Both endogenous and exogenous factors are involved in regulation of en
docrine secretions. Among the exogenous ones, light plays an important
role both in animals and in humans. Pineal gland mediates light actio
n on the endocrine system, by means of variations of melatonin (MT) se
cretion. Here we discuss about the influence of abnormal light-dark cy
cles and in particular of blindness on pineal and pituitary secretions
and on those of correlated glands. MT secretion is usually inhibited
by light: thus it reaches the highest levels at night. Exposure to sho
rt or long photoperiod causes variations in circadian or infradian MT
rhythmicity. Blind patients can show higher daytime levels with a phas
e-advanced or phase-delayed circadian rhythm. Lack of light stimulus a
ffects cortisol rhythm shifting the zenith of secretion and inducing a
free-running rhythm. Blindness can abolish nocturnal growth hormone (
GH) peak and impair the GH response to some stimuli; moreover it impai
rs the growth of affected patients. Light stimulus influences favorabl
y gonadal function both in animals and in man. In animals, sexual acti
vity and gonadal function decline during the seasons with reduced lumi
nosity. A similar finding has been described in women living in a regi
on with a strong seasonal contrast in luminosity. Blindness can impair
luteinizing hormone, follicle-stimulating hormone, prolactin and test
osterone secretion in prepubertal boys causing pubertal delay or more
severe hypogonadism; it can affect pubertal development and fertility
in women. Light can influence thyroid function in animals. Lack of lig
ht stimulus in blind man seems to cause different effects on thyroid f
unction before and after puberty. Increase of free thyroid hormone lev
els has been found in prepubertal but not in adult blind patients, pro
bably due to a resetting of the treshold for thyrotropin feedback supp
ression after puberty in these patients.