Abstract
Light-dark cycles are considered the most important synchroniser of the human circadian pacemaker. To better understand the effect of light input and the lack thereof on the circadian timing system, 12 healthy blind subjects with no light perception (NLP) were studied for ~4 months during field and laboratory conditions. In three NLP subjects who all had at least one eye, light exposure induced melatonin suppression [NLP(+); E-POS group]. In seven NLP subjects, melatonin suppression did not occur [NLP(-)]; four of them had at least one eye (E-NEG group) and three were bilaterally enucleated (BE group). In two NLP subjects melatonin suppression was not assessed [NLP(<>)]. The findings reported in this thesis show that: (1) The observed period of the melatonin rhythm measured under field conditions was not significantly longer than the observed period measured under the forced desynchrony condition in the non-entrained totally blind subjects. This shows that, contrary to previous suggestions, that average period measured in NLP blind subjects during field studies is an accurate reflection of the endogenous period. (2) The phase and amplitude of the plasma melatonin rhythm exhibited a significantly greater modulation by the sleep-wake cycle and associated stimuli in the BE group compared to that observed in the E-POS and E-NEG NLP blind groups and sighted subjects studied under the same FD conditions. This indicates that the eyes may play an additional role in the human circadian system independent of its role as a light transducer. (3) Homeostatic and circadian processes are intact and regulate sleep and waking performance in NLP blind subjects, like that observed in sighted subjects. This demonstrates that these processes function independent of conscious image-forming and non-image-forming (circadian) photoreception.