Caffeine lingers

Caffeine has a long half-life. A meaningful chunk of your 3pm coffee can still be in your system at bedtime. Even if you fall asleep fine, it can quietly reduce how deeply you rest.

The hidden loop

Poor sleep leads to more caffeine the next day, which leads to poorer sleep again. It's an easy loop to fall into without noticing.

A simple experiment

Try moving your last caffeine earlier, many people aim for early afternoon at the latest, and see how your sleep responds over a week. It's one of the highest-leverage sleep changes, and it's free.