The following article talks about how blind people have a free running circadian rhythm; in other words their wake/sleep cycle is not affected by light as with people who can see.
Below is the title and link to the article:

Entrainment of Free-Running Circadian Rhythms by Melatonin in Blind People

Robert L. Sack, M.D., Richard W. Brandes, B.S., Adam R. Kendall, B.S., and Alfred J. Lewy, M.D., Ph.D.

N Engl J Med 2000; 343:1070-1077October 12, 2000DOI: 10.1056/NEJM200010123431503
http://www.nejm.org/doi/full/10.1056/NEJM200010123431503