I understand, WalkingOnSunshine, that this statement was made in the context of the photosensitive nature of hens regarding their egg laying and duration of hours of light for egg production .... Though It's not my intent to repudiate the statement as I'm no expert in this field.... my observations in the natural world seem contrary to that statement. Living here in Sweden, it is not so difficult to travel northward to the Arctic circle and witness, where summer daylight hours are literally 24/7. If this statement is true, I am confused yet curious as to why the millions of birds of numerous species migrate north to this area of no darkness to breed, lay eggs, and raise their fledgling before returning South during the Autumn. I would have to assume that their bodies are functioning properly as that is their nature. The same would hold true for the myriad of fowl flocking seasonally to Alaska's North Slope and other northern regions of Canada, Siberia and Asia.
Having moved here over a decade earlier, from the 45° Latitude, I personally found it at first difficult to adjust my sleeping patterns during the summer period of little darkness. And though my sleeping change at first seemed slow to these new hours, it happened none-the-less and now it seems natural to me. So what I would conjecture from my personal observations and experience is contrary to the statement, however I do believe that a seasonal slowing down for R&R is good (and natural) for bodies... a rejuvenation for proper functioning. On a day to day basis though, darkness would not be a prerequisite for optimum health and vitality.
In the case of hens, it is my understanding and please correct me if I'm wrong, that it is in their nature to lay eggs (as affected by hormones) when the daylight hours reach a significant amount, as with most birds. This might possibly be a remaining ancestral link from the Red (or Grey) Junglefowl as the 14 hours of daylight would certainly signify the advent of Spring in their native habitats, a time indicative of greater amounts of insects and other food required for rearing a clutch.
Just trying to clarify...