This is rare condition but extremely disruptive to the individual. Whereas most of us have a roughly 24-hour sleep-wake cycle, in free-running sleep disorder the cycle can be become out of synch with the external environment’s cycle of day and night.
In experimental conditions when exposed to no light within the blue end of the spectrum (red bulbs are used so that the participants can still see), healthy individuals develop a free running sleep pattern.
Although this condition can affect people who are fully sighted, it often affects people who are blind. People may be blind or partially sighted but still receive the signals from daylight to help set their circadian rhythm, as the cells responsible for detecting light for circadian rhythm and those involved in sight are actually separate (1). However if the pathway from the retinal ganglion cells in the eyes, to the suprachaismatic nucleus (SCN) in the brain (the part of the brain responsible for controlling circadian rhythms) , is damaged, or if the eyes are missing, then the person may develop a free-running sleep pattern. This condition can be treated using carefully timed administration of melatonin (the body’s own sleep hormone), to entrain the circadian rhythm to match the external clock time (2).
1) Thapan, K., Arendt, J., & Skene, D. J. (2001). An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans. The Journal of Physiology, 535(1), 261–267.