>I've assumed ferrets' light receptors aren't just through their eyes, >and that even blind ones need many hours of darkness. True? The CAUSE of the blindness matters. There IS work (in Scandinavia and probably elsewhere now) in humans with different types of blindness on the amount that melatonin production is affected by light exposure. The type of blindness matters. In some types nothing gets through from the eyes in any way and those people produce more melatonin, a tumor fighter among other things, which also means less LH and less FSH produced since those go up when melatonin goes down. Chronic too high levels of LH is the usual trigger for adrenal neoplasia and chronic too much FSH can cause bone loss. There also are blind people who still have the presence of light registered hormonally if not visually, and in some blindness there can be a sensation of light. Much of blindness is a continuum with multiple types as well. It was actually due to epidemiological work in Scandinavia noticing that many blind people had much lower levels of hormonal malignancies which ultimately led to finding that melatonin is a tumor fighter. A recent study that I did not save (sorry) was on an alternate pathway rather than sensed vision (and I think through alternate minor neural pathways rather than use of the optic nerve, so I think in those of us with glaucoma even when it is fully bad) in which light was registered w hormonal results but not sensed by the people. There also was work maybe about 2 decades ago now to see if light on the backs of the knees changed melatonin levels for the worse. The study I know of where that was done found no difference. BTW, for anyone who was wondering who I forgot to tell: the surgery trial using SLT on my worse eye instead of my usual ALT got my eye pressure down enough for a while again, so that is in my arsenal now, too. Sukie (not a vet) Ferrets make the world a game. Recommended ferret health links: http://pets.groups.yahoo.com/group/ferrethealth/ http://ferrethealth.org/archive/ http://www.miamiferret.org/ http://www.ferrethealth.msu.edu/ http://www.ferretcongress.org/ http://www.trifl.org/index.shtml http://homepage.mac.com/sukie/sukiesferretlinks.html all ferret topics: http://listserv.ferretmailinglist.org/archives/ferret-search.html "All hail the procrastinators for they shall rule the world tomorrow." (2010, Steve Crandall) On change for its own sake: "You can go really fast if you just jump off the cliff." (2010, Steve Crandall) A nation is as free as the least within it. [Posted in FML 7515]