Q:

Why is coffee a stimulant?

The effects of coffee as a stimulant are quite well-known. What are the mechanisms by which coffee helps prevent sleepiness? I have heard of the presence of caffeine, but what does it actually do inside our body?

A:

Drowsiness is caused by the chemical adenosine binding to a nerve cell's adenosine receptor. A compound that binds and elicits a response, in this case a slow down of nerve activity, is called an agonist.
Caffeine is also capable of binding to the adenosine receptor. However, it does not elicit a response and no drowsiness is experienced. This type of compound, binds with no response, is called an antagonist.
When caffeine is in your system it blocks the nerve cell from "seeing" any adenosine, causing the cell to increase activity. This increase of neural activity is interpreted by the brain as an emergency resulting in the release of adrenaline, the "fight of flight" hormone.
Caffeine stops you from being drowsy, speeds up your nerve activity, and gives you an adrenaline rush. All of these interfere with sleep.

A:

Adenosine is a naturally produced hormone. It inhibits neurons and makes you tired. The longer you stay awake, more adenosine is produced.
Caffeine is a competitive inhibitor. It will bind to the adenosine receptors and prevent adenosine from binding. The adenosine is unable to bind and thus you don't get tired.
For (much) more detail take a took at the pharmacology of caffeine.

A:

I think, caffeine is one, main, but not the only-cause behind stimulant effect of coffee. 
It seemed to me, that coffee works as stimulant within moment; is due to its aroma. And after some google search I found following facts.

Caffeine is odorless:  Wikipedia (though wikipedia tells about pure dry-sample) , PubChem, ILO-ICSC.
...  so caffeine is likely not responsible for coffee aroma. 
Caffeine is not well evaporable: PubChem, PubChem
... so likely caffeine is present in very little amount in the aroma coming out from the cup.
Also; after consuming coffee, there should require certain time to onset action after consumption (Wikipedia info-box: onset of action take place at approx 1 hour, and same wiki-page another place, 45-min.). If inhalation of caffeine take place in small-amount, it should take certain time.
...but just sitting beside a cup of warm coffee ant taking smell,  stimulates me (and perhaps others too). And it is quite clear, the 'smell' is refreshing. 

... so it is quite clear that the coffee aroma-components (where possibly is negligible role of caffeine) have some part in stimulation action. Maybe that includes the signaling through olfaction.
This website gives a list of aroma compounds in coffee. 

The website tells, coffee aroma can show change in certain gene expression in rat's brain, associated with some stress-. They hyperlinked to a paper: NCBI link , PDF
The paper presented their data in followed table. 

The similar information also mention on WebMD, and some other websites like this and this
Source: Google. 

