Tag Archives: circadian

Dark Skies and Light Pollution

The mission of the International Dark-Sky Association is “to preserve and protect the night time environment and our heritage of dark skies through quality outdoor lighting.”  They’re all about stressing the importance of lighting on health, light pollution, and some really interesting stuff.

For more on the topic, check out their website, darksky.org, and Paul Bogard’s book, The End of Night: Searching for Natural Darkness in an Age of Artificial Light.

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Paleo breeding: mating in the wild.

I’ve adapted much of this chart from Howell-Skalla (2002)  and Tsubota (1998).

Canadian polar bears: bona fide seasonal breeders.circannual hormones

The light cycle increases until June, then decreases until December.  Melatonin goes in the exact opposite direction. Testosterone peaks around the onset of breeding season (springtime, April/May), coinciding with LH (as expected). There is also a lot of bear-on-bear violence at this time due to: 1) testosterone-induced aggression; and 2) the high female:male ratio –-> females rear their cubs and are thus out of the game for about 3 years, but males like to breed every year.

Females followed a similar pattern, with estrogen peaking around breeding season and prolactin following the light cycle.

The authors mentioned that prolactin levels mirrored day length, and according to Wiley this would be the prolactin peak that normally occurs when you’re sleeping, but has spilled over into the daytime due to short sleep / long light cycle… not total prolactin levels (24h AUC?), which should be highest in winter (see below).

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The incredible camping experiment, circadian proper

Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle (Wright et al., 2013)

Abstract (edited): The electric light is one of the most important human inventions. Sleep and other daily rhythms in physiology and behavior, however, evolved in the natural light-dark cycle, and electrical lighting is thought to have disrupted these rhythms. Yet how much the age of electrical lighting has altered the human circadian clock is unknown. Here we show that electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day, increased light exposure after sunset, and a delayed timing of the circadian clock as compared to a summer natural 14 hr 40 min:9 hr 20 min light-dark cycle camping. Furthermore, we find that after exposure to only natural light, the internal circadian clock synchronizes to solar time such that the beginning of the internal biological night occurs at sunset and the end of the internal biological night occurs before wake time just after sunrise

In other words, they compared circadian events during 2 weeks of normal life to 2 weeks of 100% camping.  And camping won.

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