Human Cone Light Sensitivity and Melatonin Rhythms Following 24-hour Continuous Illumination

Abstract

This study investigates the possibility of an endogenous circadian rhythm in retinal cone function in humans. A full-field cone electroretinogram (ERG) was performed every 2 h for 24 h under continuous rod-saturating ambient white light (53 ± 30 lux; pupils dilated) in nine healthy subjects. Distinct circadian variations were superimposed upon a gradual decrease in cone responsiveness to light, demonstrated most reliably in the implicit times of b-wave and oscillatory potentials, and to a lesser extent in amplitude and a-wave implicit times. After mathematical correction of the linear trend, the cone response was found to be greatest around 20:00 h and least around 06:00 h. The phase of the ERG circadian rhythm was not synchronized with the phase of the salivary melatonin rhythm measured the previous evening. Melatonin levels measured under constant light on the day of ERG assessments were suppressed by 53% on average compared to melatonin profiles obtained previously under near-total darkness in seven participants. The progressive decline in cone responsiveness to light over the 24 h may reflect an adaptation of the cone-driven retinal system to constant light, although the mechanism is unclear. The endogenous rhythm of cone responsiveness to light may be used as an additional index of central or retinal circadian clock time. (Author correspondence: [email protected])

Keywords:

• Circadian rhythm
• 24-h constant light
• Human cone electroretinography (ERG)
• Melatonin

ACKNOWLEDGMENTS

This work was supported by the European Commission (FP6 IP EUCLOCK). We thank Mark Hankins and Rob Lucas for initially proposing the protocol of this study. We are grateful to the study participants, Elena Danilenko, Natalia Danilenko, and Sergei Kondratov for their help in preparation/running the study, and Jakob Weber and Ekaterina Semenova for melatonin assays.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.