My Speculation Du Jour:
I have wondered if blind people are effected by Circadian rhythms as we sighted people are? I speculate that yes, their body is effected by light, the light that is absorbed from around them. Light can easily pass through our skin and other membranes, thus effecting the organs (guts) of the body system.
They are affected, from cells receiving light in their eyes.
Also, the moon may help, too.