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I tried reading by light from a neighbouring room and felt sleepier than if my own lights were on.

Is there a formula for the intensity of light vs physiological preparation for sleep(calcium, melatonin)? Or is there a threshold intensity below which the body goes to "sleepy mode"?

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I don't believe either is the reason. Back in the day of early man, the different intensity levels of light was used as a signal to our body's "master clock" to tell us when it was time to go to sleep. So to simply put it, when exposed to less intense light, such as artificial light, its picked up as the dimmer, less intense, light akin to when historically we would be getting ready to go to sleep.

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  • $\begingroup$ There are new studies that are challenging this hypothesis, but there is some degree of speculation when trying to work out biochemical reactions of our prehistoric ancestors and how those genes affect us today. $\endgroup$ – AMR Oct 30 '15 at 17:22
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It appears that short wavelength light (that is light around the blue and purple end of the electromagnetic spectrum) does have a significant effect on melatonin levels and the 'alerting response'.

Exposure to 2 h of monochromatic light at 460 nm in the late evening induced a significantly greater melatonin suppression than occurred with 550-nm monochromatic ligh - See more at: http://press.endocrine.org/doi/abs/10.1210/jc.2004-0957#sthash.feusLJf4.dpuf

Note that 450nm wavelength light is blue and 550nm light is yellow.

Interestingly, there is some evidence that occular photoreceptors (being neither rods nor cones) play a role in setting the circadian clock. These researchers hypothesize that retinal ganglion cells (nerve cells) are photo-receactive and play a role in regulating the circadian clock via innervation of the hypothalamus in the central nervous system.

Light synchronizes mammalian circadian rhythms with environmental time by modulating retinal input to the circadian pacemaker—the suprachiasmatic nucleus (SCN) of the hypothalamus. Such photic entrainment requires neither rods nor cones, the only known retinal photoreceptors. Here, we show that retinal ganglion cells innervating the SCN are intrinsically photosensitive. Unlike other ganglion cells, they depolarized in response to light even when all synaptic input from rods and cones was blocked. - Science 8 February 2002: Vol. 295 no. 5557 pp. 1070-1073 DOI: 10.1126/science.1067262 http://www.sciencemag.org/content/295/5557/1070.short

So, while there is no 'formula' per-se, we might conclude that higher energy wavelengths (that is shorter wavelengths near blue and purple) are more effective in suppressing melatonin and more effecting in keeping you awake, as opposed to lower energy wavelengths like yellow and red.

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