Of the six circadian-related sleep disorders listed in the ICSD two have particular relevance to the blind: (i) free-running sleep disorder, in which the circadian clock is not entrained to the 24-hour day but ‘free-runs’ with an endogenous period somewhat longer than 24 hours (on average) [1], [2], [3] and (ii) abnormally entrained sleep disorder, in which sleep is entrained to 24 hours but at an incorrect phase angle i.e. sufferers wake either very early (advanced sleep phase disorder, ASPD) or very late (delayed sleep phase disorder, DSPD) with respect to societal norms. The gene discussed is CLOCK; the disease is sleep disorder.