We analyzed 37,000 nights of wearable data to see what caffeine actually does to your sleep

Most people think caffeine keeps you awake. It doesn't — caffeine had no meaningful effect on time spent lying awake in bed (p-value ~0.9).

What it actually does is destroy your REM sleep while leaving light sleep almost untouched. The damage is invisible — you fall asleep fine, but the most restorative part of your sleep takes the hit.

Full analysis: Caffeine Kills Your Deepest Sleep First