“FREE- RUNNING” CIRCADIAN RHYTHM
Most totally blind people have free-running circadian rhythm. (i.e. Not synchronized to 24 hr light dark cycle, but oscillates on a cycle slightly longer than 24 hr.).This causes insomnia and daytime sleepiness. A similar phenomenon is seen in subjects exposed to continuous absence of natural light in Antarctica during winter months.
Melatonin secretion shows a circadian rhythm, the level being higher at night, the peak at 3-4 am. This fluctuation depends on the sympathetic innervations of the pineal gland. (Nervi conarii).Further in blind melatonin level shows an approximate 24 hr rhythm, but the peak may be at any time. So, in blind the biological clock (SCN of Hypothalamus) is able to maintain circadian rhythmic secretion but not its relationship to light-dark cycle. Such a 24hr rhythm which shows no relation to light or dark is called “free-running”.since evidences support that, diurnal changes in melatonin secretion function as a timing signal that coordinates endocrine and other internal events with light-dark cycle, the blind people can be “ entrained”to 24 hr cycle with melatonin.
Note: Retino hypothalamic fibers synapse in the SCN. There are connections from SCN to intermediolateral gray column of spinal cord.preganglionic neurons pass from spinal cord to superior cervical ganglion, post ganglion neurons project from this ganglion to pineal in the nervi conarii.
