redmonkees t1_isyq2fr wrote on October 19, 2022 at 5:55 PM

So, what you’re talking about specifically is the principle of entrainment. Organisms that use light to determine the length of sleep have a chemically based circadian clock where a protein degrades with light exposure and causes the “clock” to send out a wake signal, and vice versa for dark. These organisms can entrain to any length of day, given a certain period of light/dark cycle. You can shorten the sleep cycle by exposing organisms to a shorter day/night cycle, and lengthen it via a longer day/night cycle. Humans can adapt to a 48 hour day, in fact we have done so artificially as some have already mentioned, but I think it should be made clear that the human biological clock as it stands does not change as a result of that, it still naturally has an approximately 24 hour period (23-25 hours depending on sleep behavior across species). 48 hours actually works pretty well for us because biotic functions that work in a 24 hour clock can still synchronize fairly well with that multiple of 24. Getting away from that multiple can push some biological functions with 24 hour periods out of synch with the natural rhythm, but we will still have sleep cycles that match the light cycle because of entrainment.

Now, if we had originated and or evolved on a planet with a 48 hour day, that would be a different story, though I think that’s obvious enough. Theoretically if humans from earth lived on this planet for generations we would eventually evolve a natural clock that aligned with the 48 hour period of the sun.

nobrow t1_iszf6e8 wrote on October 19, 2022 at 8:34 PM

Where are these proteins located? In the eye?

redmonkees t1_iszpnz8 wrote on October 19, 2022 at 9:42 PM

In the eye there are photo receptors that respond to light, I think that much is obvious, but more specifically there are specific types of photoreceptive cells called retinal ganglion cells (RGCs) which contain melanopsin, a pigment that is energized by a certain wavelength of light. That energizing causes the the RGC to send a neural pulse, called an action potential to a region of the brain called the suprachiasmatic nucleus (SCN), located near the hypothalamus. Now, interestingly while RGCs do play a role in sight, this specific pathway to the SCN is different than the pathway to the optical cortex(which is at the back of the head, not near the hypothalamus), and is why some blind people can still respond to light and maintain a circadian rhythm without being able to actually see or process what they are seeing.

Now, this is complex and it’s been a minute since I studied the specific pathway, so I’ll explain it incredibly simply. Once the action potential reaches the SCN, it innervates cells there, which is where the proteins I mentioned are created. Those proteins act as transcription factors, which cause the transcription of many genes essential for sleep, such as melatonin. And I think that’s where I’ll leave it, as I’ll get it too convoluted, and I’ve probably already said some things wrong. Just know that’s generally how the system functions, and that it’s the SCN that acts as the hub which releases hormonal signals to synchronize the entire body’s cells individual clocks.

If you’re interested in the subject, there are much more in depth articles and videos on the circadian clock I’d recommend looking into.

nobrow t1_it00tz8 wrote on October 19, 2022 at 11:01 PM

That's super interesting thank you for the response. Glad you mentioned blind people because that was gonna be my next question. I googled it and it turns out blind people without any light sensitivity do suffer from messed up circadian rhythms. Wild how complex and specific the processes of our bodies are. Always blows my mind when I learn about a new one.