Adenosine is a neurotransmitter that your body naturally produces. It gets used in a lot of places, but one of those is in making you feel sleepy. Over time, it builds up and starts slowing down neurological processes, making you feel sluggish and sleepy.
When you sleep, the adenosine gets removed.
If you sleep for just a few minutes, you’re essentially taking the edge off without really solving the problem of too much adenosine. If you nap for 20 minutes, you take away a lot more adenosine. 30 minutes is pretty much the upper limit, because after that, your brain waves start slowing down a bit and you need some time to wake up afterwards (the groggy feeling). At that point, the next optimal nap length is 90 minutes because you complete one sleep cycle in that time.
Caffeine works primarily by blocking adenosine receptors, but once it fades away, you suddenly have all that adenosine that it had been holding back rush in to the receptors. This is a caffeine crash.
So, here’s how caffeine naps work
You make a very strong cup of coffee (usually I go with two espresso shots) and drink it as fast as you can without burning yourself. 10 minutes is your target. Set your timer for 20 minutes (plus 5-10 minutes to fall asleep), and sleep in a dark room. When you wake up, you’ll feel great from the reduced adenosine levels. And since caffeine takes — 45 minutes to work, you’ll feel even more awake, since it’s blocking what adenosine is left.
If that isn’t an option for staying awake, one thing I’ve found that works is licking the roof of the mouth. It feels really weird, and will make you alert for a few more seconds with how unpleasant it feels. Not really suited to long lectures, but good for short (10-15 minute) meetings.