It does, but not in the way you might think.
Caffeine alone doesn't actually do much for mental alertness beyond a bit of a boost in awareness early on after drinking it. On its own, caffeine induces "paradoxical effects" that involve both stimulation and relaxation at different times, which can make it difficult to concentrate or even easily fall asleep later on. The wakefulness-promotion effects of caffeine are only evident when consumed with carbs – often sugar – as this provides glucose to eliminate the initial paradoxical effect of too much stimulation from caffeine without carbs and an energy-deficit from a lack off carbs.
Do you want to feel fully energized and refreshed, or just a little awake?