That's a reference to this study (poor rats!): LinkUm, does this mean that if you have light sensitivity (or damage to retinal photoreceptors) that taking melatonin makes it worse?
So, if I understand correctly, in simple terms, melatonin injections before the rats were exposed to a lot of light, led to additional eye damage. But melatonin injections after the exposure of light led to no additional eye damage. Which may translate to: Don't take melatonin in the morning or maybe even early afternoon (but midafternoon is okay as mentioned earlier). Or: If you're planning to look straight at the sun or staying in a very bright place, you might want to take melatonin later! It can be tricky translating such studies.Melatonin increases photoreceptor susceptibility to light-induced damage.
Melatonin is an indolamine hormone synthesized in the retina and pineal gland. It is thought to act as a paracrine neurohormone in the mammalian retina. Pinealectomy has been shown to protect photoreceptors from light-induced damage, and melatonin treatment has been reported to increase the degree of photoreceptor damage in albino rats. To determine how melatonin influences photoreceptor survival, the effect of melatonin administration on light-induced retinal damage was studied. Melatonin was administered to albino rats by intraperitoneal injections at various times before or after light exposure. The rats were exposed to high-intensity illumination (1600 lux) for 24 hr to induce photodamage, then returned to cyclic lighting for 12 days. After this, they were killed, and their eyes were removed and examined histologically. Measurements of the outer nuclear layer (ONL) thickness were taken at 12 different loci around the circumference of the retinal sections. The animals that received daily melatonin injections (100 micrograms) in the late afternoon (3 hr before lights off) for 1-3 days before photodamage showed an approximate 30% greater reduction compared with sham control animals in ONL thickness in the superior quadrant, the area most susceptible to light damage. Melatonin injections given after the photodamage did not affect ONL thickness. Although retinal susceptibility to light damage varied with time of day, the degree to which melatonin increased the degree of damage appeared unaffected by the time of day. These results suggest that melatonin may be involved in some aspects of photoreceptor sensitivity to light damage.
FWIW: I have corneal scar tissue, and I used to be very sensitive to light, leading me to wear sunglasses most of the time, but that went away at some point and now I only have to do it on some summer days when it's very light out there. I'm saying this because I regularly took/take melatonin for periods of time one to several hours before sleep, and it never affected my sensitivity in a negative way. I doubt that a high concentration of melatonin would. Though perhaps (just a guess though!) taking it late afternoon and/or evening (if single dose; evening) would be the safe route if you have high sensitivity to light.
Hope this helps!