A 6pm caffeine cutoff is really very late. Most people would cutoff no later than 2 or 3. Not saying you’re not a natural night owl, but your approaching to caffeine is very likely not allowing you to get a clear picture of your natural cycle.
I can drink coffee after dinner and go to sleep almost immediately and feel great when I wake up. Coffee does literally nothing to my ability to sleep. It depends on the person.
I thought caffeine was only active for 4-5 hours, unless you have a particular gene mutation that causes slow metabolism?
That’s the half-life. After that time it will be half gone, so the remaining half will still be active for a while.