Sleep and circadian phenotype in people without cone-mediated vision

Background: Light exposure entrains the circadian clock through the intrinsically photosensitive retinal ganglion cells, which sense light in addition to the cones and rods. In congenital achromatopsia (ACHM; prevalence 1:30-50,000), the cone system is non-functional, resulting in severe light avoidance and photophobia at daytime light levels. How this condition affects circadian and neuroendocrine responses to light is not known. Methods: In genetically confirmed ACHM patients (n=7; age 30-72 years; 6 women, 1 male), we examined survey-assessed sleep/circadian phenotype (PSQI, ESS, MEQ, MCTQ), self-reported visual function (NEI-VFQ-25), sensitivity to light (VLSQ-8) and use of spectral filters that modify chronic light exposure. In all but one patient, we measured rest-activity cycles using actigraphy over 3 weeks and measured the melatonin phase angle of entrainment using the dim-light melatonin onset (DLMO). Results: ACHM patients experience a severely attenuated light-dark cycle due to severe light sensitivity and habitual use of filters to reduce retinal illumination. In aggregate, both MEQ and MCTQ indicated a tendency to late chronotype. We found regular rest-activity patterns in all patients and normal phase angles of entrainment in participants with a measurable DLMO. Conclusions: Our results reveal that a functional cone system and exposure to daytime light intensities are not necessary for regular behavioural and hormonal entrainment, even when survey-assessed sleep and circadian phenotype indicated a tendency for a late chronotype and sleep problems in our ACHM cohort. Our results can be explained by an adaptation mechanism in circadian photoreception which adjusts to the range of habitual light exposures.