So what does drinking coffee actually do?

[simsfan]

It doesn't fill up the energy bar at all so what does it do?

[Saltypaws]

That is weird, it should, at least a little.  I know the expresso machine gives them some energy, not alot though.  Has to make them use the restroom though, lol.  I can't remember now, if it boosts their fun level, I know it did in sims2.

[Sportsfan]

It freezes it for a while I think

[Seabody]

I've played the console version once, and in that time, my Sim never drank coffee.
In the PC version, however, it gives them a small moodlet for a few hours. When that expires, they get a negative moodlet for a few more hours.

[simsfan]

All it does that I know of is it gives me a moodlet when I drink coffee and then it gives me another moodlet once I get the caffeine crash.

[Pam]

In my experience on the PC version, it also keeps the Sim from falling asleep even if he's sleepy.  I don't let my Sims drink it.

[Captain]

My experience is also only with PC, but drinking coffee every 3 hours pretty much lets your sim go without sleep indefinitely, with only a hit to mood when the crash comes, but it's overwritten if you drink another coffee.

A decent tactic for a challenge if you need to get things done fast and don't have LTH points for the motive mobile.

[simsfan]

Last time I drank it my energy went down like I never drank it at all.

[Captain]

It shows your energy down, it doesn't move the actual bar up at all.  But it gives a "Buzzed" moodlet and removes any very sleepy moodlet you may have, so you can continue doing whatever you need normally with no fear of your sim complaining or passing out.  After 3 hours the buzzed moodlet goes away you get the crash and the sleepy moodlets, but drink more coffee and they are again replaced with buzzed so you can keep going.

At least on PC.  I have it on the 360 but don't have time at the moment to check this.  Is that the behavior you're seeing?