I'd be interested in an elaboration on how you assert the person who wakes up every morning can be certain they're the same person who went to sleep the night before.
The notion that we might not be comes up in multiple places, but is largely an extrapolation of the Transporter Paradox, in which continuity is the only known link we have between some things in two states (as per the Ship of Theseus). AI programmers contemplate it when they have to reboot their test subject (which are related to, but not the same as LLMs or Generative AI projects, rather are efforts towards creating AGI). When an AI is rebooted, is it the same entity as it was beforehand? In the webcomic Freefall this is considered by robots, and while a large bloc of robots are not keen on upgrades. Mark Stanley gets deep into the discussion within the comic
CGP Grey noted in his Transporter Paradox video that sleep might be the same as a transporter event since the brain's cerebellum shuts down to a state of unconsciousness in NREM sleep (SWS sleep) and in fact, as old people approach death they experience increasing amounts of NREM sleep until, if they are lucky, they just don't wake up.
exurb1a's video Sleep is just death being shy is a philosophical look at this phenomenon.
So yeah, without any kind of established spiritual phenomenon (for which there is absolute zero evidence -- we've checked at length) the only thing linking who you are when you wake up, and who you were when you went to sleep is the consistency of the material world matching (more or less) the memories of the person waking, which gets weirder when unconsciousness extends longer than a night's rest (such as going unconscious due to anesthetic or a coma state).
Who we are is a very ephemeral state, a quasi-stable event. And we exist longer than a day of consciousness only because we define our narratives that way. And some creators like Phillip K. Dick have notoriously raised challenges to this by offering narratives in which continuity and identity are unreliable.