PhotoperiodEffect.com

6. We've been extending our days with extra light forever. If extending our hours by more light is so harmful, wouldn't the taming of fire by Homo Erectus a few hundred thousand years ago have done us in then, or woudn't you think we'd have adapted to it by now?

The amount of light from firelight is fairly small, and in times past fires in the night posed the extreme danger of pinpointing one's location for raiding parties. Most frequently, for example, native peoples in North America heated rocks in fires before sunset and took those to bed for extra warmth during the night. More significantly, the special ipRGC cells in our eyes that tell our brain if it's day from night don't respond to the relatively low (“soft” or “warm”) red and orange frequencies most typical of a low fire. More than one plausible explanation can be put forward for this. No doubt detecting red light is sufficient to tell day from night, but also the proportions of red light vary during the day, red light is most typical of the beginning and end of the day, and/or because physiologically, detecting only higher frequencies of light may be easiest since blue photons pack more punch than red ones, more easily changing proteins; or the process of detecting red may require less expenditure of energy overall or consume less Vitamin D (ipRGCs are in fact highly resistant to Vitamin D depletion). Modern versions of these ancient cells might possibly be shifted even further toward the lower, red end of the spectrum due to longstanding exposure to firelight past sunset for thousands of generations, but this isn't known, or necessarily so. More likely, it was simply the good fortune of Homo Erectus that the particular technological innovation of domestic fire didn't significantly confuse the body's sense of night and day as it was already the case that ipRGCs were not tuned to detect reddish light. Perhaps only the advent of candles, and efficient oil lamps, began to cause any significant additional stimulation for our ipRGCs; and only once gas and electric light vastly increased the brightness and dramatically reduced the cost of illumination did the era of modern chronic illnesses truly begin – although historical economic analyses show the increase in illumination for the European population as a whole before and after 1800 to be a much smoother logarithmic increase than this suggests.