Melatonin is the hormone that gives the "expectation" of darkness. It has no use in blind people, so do they slowly stop (or immediately) secreting it? I heard it is given artificially to help with circadian patterns.
Before answering your question, let’s first go through some essential concepts:
What are the structures involved in the circadian system? The organ that controls the 24-hour rhythm responsible for sleep-wake cycles, alertness and performance patterns, fluctuation in body temperature and production of hormones (e.g melatonin and cortisol) is the suprachiasmatic nuclei (SNC) of the anterior hypothalamus. The major environmental factor that resets our internal clock to 24 hours is the light and dark cycle generated by the earth’s axial rotation.
The neuroanatomy of the circadian system is following: light information is transduced from specialised retinal photoreceptors to the SCN by the retinohypothalamic tract (RHT). SNC projects to the pineal gland via the paraventricular nucleus (PVN) and the superior cervical ganglion (SCG). Under normal light-dark conditions, the pineal melatonin rhythm peaks at night during the dark phase. However, light exposure during the night inhibits melatonin production.
[Melatonin] has no use in blind people, so do they slowly stop secreting it?
Interestingly, several studies have tried to monitor the level of cortisol and melatonin in blind people and the results were very different. Overall, there is a heterogeneous distribution of melatonin rhythm types. According to one of the longitudinal study conducted among 20 blind subjects:
Three subjects had normally phased melatonin rhythms, three were abnormally entrained, and 11 had free-running melatonin rhythms with periods ranging from 23.86 to 25.08 h. The remaining three subjects appeared to be arrhythmic.
Why is the case? Probably because not all disorders of the visual system cause a disruption of the circadian effects of light. Several studies have shown that the majority of legally blind individuals retain some degree of light perception even with very little usable vision.
Why do individuals with absent or attenuated rod and cone function still retain circadian responses to light (20% of the nonperception of light patients according to some studies)? Some recent advances have identified a new non-cone photoreceptor system (based on a novel molecule called melanopsin) that is involved in circadian rhythm regulation.
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