neuroscience - What light intensity determines the start/end of a photoperiod in humans?

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^[1].

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?

1. Ribelayga C, Pévet O, Simonneaux V. 2000. HIOMT drives the photoperiodic changes in the amplitude of the melatonin peak of the Siberian hamster. Regulatory, Integrative, and Comparative Physiology, 278(5), R1339-R1345.

Answer

It isn't really a specific light intensity that an organism will interpret as the start of a photoperiod. Maynard Johnson reported in 1939 that intensity of light used had an effect on the rate at which mice would shift their circadian rhythms. So, brighter light has a stronger effect, but there isn't a cutoff point that I'm aware of. Johnson was using illumination as low as 2.5 foot candles, which is pretty dim. Aschoff and Pittendrigh did some more work on light intensity in the 50s and 60s, but I don't think anyone ever found anything resembling a cutoff point, only the degree to which intensity had an effect.

Computer monitors are pretty bright. Luminance rating is typically 250-350 cd/m2, which, for comparison with Maynard's experiment, is eqivalent to 23-32 foot-candles. http://computer.howstuffworks.com/monitor6.htm

Johnson, Maynard S. "Effect of continuous light on periodic spontaneous activity of white‐footed mice (Peromyscus)." Journal of Experimental Zoology 82.2 (1939): 315-328.