I believe that for many people biphasic (interrupted) sleep is natural. https://en.wikipedia.org/wiki/Biphasic_and_polyphasic_sleep#:~:text=Biphasic%20sleep%20(

I believe that for many people biphasic (interrupted) sleep is natural. https://en.wikipedia.org/wiki/Biphasic_and_polyphasic_sleep#:~:text=Biphasic%20sleep%20(or%20diphasic%2C%20bifurcated,of%20sleep%20within%2024%20hours https://en.wikipedia.org/wiki/Biphasic_and_polyphasic_sleep#.... I think I used to sleep for more like 6-8 hours but these days I always wake up after around 4 hours and then answer messages or look up random things for an hour or so and then go back to sleep for another 3-4 hours. I suspect part of it might be that I have to urinate more or something so don't stay asleep.But maybe it's just natural. One other obvious thing is caffeine. It can keep you awake for like 12 hours or more even if you aren't otherwise feeling particularly stimulated anymore. But it sounds quite serious so I think you should get some specialist medical help if those things don't resolve it.