I don't think it's anything confounding; it's likely just age. Old people are really sensitive about drinking caffeinated coffee late in the day for reasons I never understood until my mid-30s, where suddenly I had to watch soda intake after 6pm or I'd get insomnia.

(Also, use phone/monitor night mode and don't do anything intense within 1hr of bed-- no gaming, no exercise, no forums, no outrage-inducing news, etc. Wind down with something boring.)