How a lack of light perception affects sleep patterns.
Light and dark, day and night - all affect our circadian rhythms, which in turn govern our sleeping patterns, energy levels and moods.
What happens if you can't see? An estimated 50% of all blind and visually impaired people have some kind of sleep disorder, and many more suffer severe sleep problems due to this lack of light perception.
Ian Hamilton investigates the problem and explores the possibility of a cure.