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.