I'm reading this article, which discusses the influence of Long Photoperiod (LP) and Short Photoperiod (SP) on melatonin production: HIOMT drives the photoperiodic changes in the amplitude of the melatonin peak of the Siberian hamster.
I'm interested in knowing what light intensity is interpreted as the onset and end of a light-dark photoperiod.
The referenced article indicates that animals were kept in cages, with a pre-set photoperiod duration, this sounds to me like it was done indoors. If it has been done indoors, then artificial lighting was involved, and I would guess it was an on-off affair, rather than gradual change or jumps in intensity.
The reason why I'm asking is that I'm trying to understand the implications of this article for humans, who may wake up before dawn, and spend some time under artificial lighting of various intensity before sunrise, and continue to function after sunset. For example a person may be looking at a TV or a monitor, and I would like to get an idea if this "presence of light" is enough to trigger photoperiod-related changes in the brain. Is there some "critical" density (or density at a specific wavelength) of light that signals the start/end of a photoperiod?