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

For a really great interactive online chart exploring the different stages of sleep, visit: http://www.pbs.org/wgbh/nova/body/sleep-memory.html  and click on “LAUNCH INTERACTIVE”.

What happens in your brain as you cycle through the various stages of sleep, and how does this activity affect learning and memory? See what scientists are learning about REM (rapid-eye movement) sleep and other phases, and explore recent research linking sleep—and sleep deprivation—to different types of memories.

Explore the stages of a good night’s sleep and the research linking sleep to memory.