Summary of a presentation by Orie Shafer titled "Circadian timekeeping and entrainment in neuronal clock networks."
From the classic Wever study, which put participants in underground dungeons to study circadian rhythms in the absence of sunlight...
and showed there are multiple inputs, both external and internal zeitgebers.
In this study, in the absence of sunlight, the human circadian rhythm in locomotor activity, bed movements, and rectal temperature was about 25.8 hours:
The study wasn't perfect, but still cool.
Most people like to simplify and say light is the input, and while it may be the main one, we now know there are also other important inputs such as food intake for the food-entrainable oscillator and exercise for the skeletal muscle clock.
"Behavorial rhythms are driven by molecular rhythms. Molecular clocks are required in small islands of the brain for behavorial, endocrine, and physiological rhythms." Search for genetically modified models of virtually any clock gene and it's going to influence a wide variety of processes. Like, circadian rhythms are important for certain aspects of nearly everything. "60% of the time, it works every time" lol
Very basic figure:
I say that because some of these things can easily be manipulated, for example, studies have shown that habitually exercising in the morning deletes the morning/afternoon difference in performance.
The science on this (below), too, is evolving:
It was once thought that some humans were morning larks whereas others were night owls. It turns out that studies on this show 'chronotype' is more of a species-level phenomena -- that is, humans are morning chronotypes, mice and owls are evening chronotypes. Morning-people do better on a wide variety of physiological and cognitive outcomes, also healthier, than night-people.
Average sleep duration is 8 hours with midpoint at around 4:30 am meaning sleep is from ~12:30 am to ~8:30 am. On average.
But check this out: I've been criticized for saying 8 hours, seasonally, may not be optimal (opting for more, even up to 9.5 based on Wiley's rationale). We're chronically socially jet lagged (circadian phase delayed):
If we let people sleep naturally in an environment which discourages social jet lag, BOOM, 9 hours!
...
There were some points made suggesting a causal relationship between social jet lag and smoking. I don't know, but instantly thought about the high number of smokers among shift-working nurses.
...
This doesn't bode well:
...
Interestingly and not surprising given the above findings, under electrical lighting conditions melatonin onset is more variable and highly delayed compared to that under natural light:
Below: top figures: good. Adequate sleep confined to the dark phase. Bottom figures: not so good. Reduced sleep on weekdays and 'catch-up sleep' on the weekends.
That's all for now!
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