I end up lying awake until 4 am and then have some very very light sleep for about and hour or two. My Garmin shows me that I get no deep sleep at all.

If it came on sudden I would say look for changes in your direct environment. Something dramatic? Did you maybe hurt yourself? Did you start eating a new food? Someone pop back up in your life that you thought was gone? That sort of thing. If you can not bisect the issue a sleep study would be better. Medication may help but it may mask the underlying issue.

For me it was more of a realization it was my diet. I changed my diet to remove some particular types of foods and now I sleep good. If I eat those foods I sleep like crap. Pop a tums and I am good (it is the upset stomach waking me randomly, so the bad sleep). Everyone has different reasons why. But the top ones are typically stress, injury, or food are a good place to start.

check this out - https://insomniasos.net/, and Huberman's guide on sleep - https://www.youtube.com/watch?v=nm1TxQj9IsQ

Any naps during daytime?

No - I’ve never been able to nap.

Just my personal opinion: go to a doctor with a sleep laboratory and get consultation there. Also if I were you I would try to get a brain CT scan.