It actually a fairly large leap, considering that caffeine can impair sleep quality, causing more fatigue or fatigued people could be consuming caffeine to reduce the fatigue.
In many people, both may be happening, so teasing out the direction of causation is difficult.
The two go really well together. I recently quit coffee and started sleeping 10 hours a night. It's hard to get to sleep on time when I've had coffee, but if I get enough sleep, I don't need to have coffee to feel awake.
Exactly—-and there’s probably a fairly vicious feedback loop too. Tired people reach for caffeine, rebound effects (or mistimed doses) make them even more tired. Lather, rinse, and repeat....
This makes it hard to figure out if the obvious intervention (ban energy drinks) would actually do anything, or if the answer is really more rest—an idea the military has been loathe to adopt in general.