Melatonin is an interesting hormone.
Secreted by the pineal gland, the neurohormone is found in higher serum concentrations during the hours of darkness and when levels of the neurotransmitter dopamine are reduced. The opposite is the case during daylight hours and it seems that this balance is an underlying driver for the circadian rhythms that govern our metabolic processes. Those that are so easily disrupted by modern life – jet lag, poor health linked to odd working patterns under constant artificial light, seasonal affective disorder are all in various ways linked.
Some time ago I was lucky enough to interview Professor Russell Foster who had first noticed how cortically blind mice were still able to exhibit signs of diurnal behaviour patterns. When he first postulated an additional retinal photoreceptor to rods and cones, he was dismissed offhand by colleagues.
As is often the case with inspired researchers, he has since been vindicated and it is now known that the retina has light receptive ganglion cells which transmit information about external light levels directly to the hypothalamus region to help regulate the body clock.
A new study from Ulster University has now shown that melatonin serum levels are higher in myopic humans. Animal studies have indicated that melatonin has some influence on tissue growth, such as axial length increase, so it may well be that the retardation of myopic progression shown with increased daylight exposure for susceptible children may have an underlying physiological explanation.
All those new schools being built across China with some classrooms completely open to the outside might well be justified if the increased light levels reduce melatonin production and therefore reduce the stimulus for axial elongation.
Yet another fact to add when discussing myopia with worried parents – perhaps as you are explaining to them about the new myopia ‘calculator’ just released by the Brien Holden Institute (https://calculator.brienholdenvision.org). But more on this next week…