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.

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.

The people suggesting that we have a 48 hour sleep cycle aren’t completely right. The 48 hour cycle works because they were still exposed to artificial light when they wanted to be active. 48 being a multiple of 24, there was still overlap with biological functions and it’s an easy rhythm to entrain. Without light altogether diurnal organisms naturally fall into closer to a 25 hour cycle.

Kind of sort of. It was a misleading study and didn’t fully understand the mechanisms behind sleep. They were devoid of sunlight, but not light altogether. They still used artificial light, which disrupted the degradation of proteins in the circadian clock which normally would signal you to sleep during periods of darkness. They essentially deprived their bodies of sleep by extending the period where the signal to stay up (light) was present. While a 48 hour period can be entrained, biological assays have shown that there is still a biologically maintained free running period of around 25 hours throughout the body. This is present in almost all diurnal organisms (organisms that are active during the day). Nocturnal organisms tend to have a free running period closer to 23 hours. Both of these have been observed in environments completely devoid of all light, and also in blind humans with complete enucleation (lack of eyeballs), who are unable to have any photo reception. The 48 hour period works because it is a natural multiple of 24, and allows some synchronicity with biological functions throughout the body that were determined by the 25 hour free running circadian clock. While we can function under those conditions, and will entrain to that period, by no means is our biological determined sleep cycle specifically 48 hours.

Our Human circadian system (and most vertebrates and many inverts, though often through different biological mechanisms) is completely governed by light availability. Light presence degrades a protein that is a part of our circadian “clock”, and sets a rhythm that tells us to sleep. You can set a circadian rhythm in humans with just an hour of light exposure at the beginning and end of the day time. Interestingly, hibernation and different types of torpor (prolonged moments of rest) are totally unlinked to the circadian system, which solely governs our sleep based on the position of sun in the sky/daylight. When you think about it, that makes sense - they often go into burrows, where sunlight reaches very little. Hibernation is actually more so linked to cold; though whether that’s a direct link to cold or a result of decreased nutrient availability varies between species. It been shown that some species at the very least actually are able to complete shut down their circadian system from functioning during hibernation to prevent interference with the two systems.

A really interesting part of animal life in the arctic circle though is that not all species hibernate. Arctic reindeer, which don’t hibernate, have been observed to have a much less controlling circadian system compared to closely related, non arctic species. They’ve essentially shut off their circadian system as an adaptation to maintain essential biological needs throughout the day. They are able to graze periodically every few hours throughout the day as their body needs, even without the presence of sunlight. They do still have higher prevalence of melatonin during the night, which indicates that they can still denote when the seasons change, but it’s not linked to a distinct sleep period, unlike other ruminates. Unfortunately, that is an adaptation to the environment that would take generations and generations to arise in a species randomly, and as humans are relatively new to the arctic environment in an evolutionary sense, that adaptation has not been observed yet in native human populations of the arctic. Humans are still beholden to the circadian system in the arctic, meaning that in periods of full dark there is more pressure to sleep in a free running period of 25 hours.

Also, because is was mentioned, the 25 hour free running period you spoke of (free running means the natural period of sleep observed in no-light conditions) is something that has been observed in many species. It’s not actually odd like you said, because when you arrive at the mechanic behind it the answer makes sense. The link is that it’s only present in diurnal (active during the day) species. Nocturnal species tend to have a 23 hour free running period. Because the protein that signals sleep degrades in light, and that degradation takes time, this allows diurnal and nocturnal animals to be awake at the right time when entrained to the sunlight, sometime around daybreak and sometime after sunset respectively. Theoretically in some exoplanet that might have a 20 hour day, species that evolved there with the same circadian rhythms would have a 21 and 19 hour free running period because of that.