So here's a funny thing. A couple of years ago I discovered this technique to refresh myself in the early afternoon. A coffee nap is a technique where one drinks a small amount of caffeine and then takes a short nap, of 15 to 20 minutes duration. The idea is that caffeine takes that long to begin affecting your system, so you wake up just as it starts to kick in.
Caffeine works by blocking a neurotransmitter called adenosine. Adenosine is a naturally occurring chemical in the body that promotes feelings of sleepiness. Throughout the day, adenosine levels build, signaling to the body that it is time to sleep. During sleep, adenosine is cleared from the brain. So during the nap, your brain clears some of the adenosine that contributes to feelings of tiredness, leaving more room for the incoming caffeine to block the remaining adenosine receptors. This 15--.20 minute nap timing therefore creates a stronger boost in alertness than coffee or a nap alone.
I occasionally use a coffee nap to fight midday fatigue and improve focus. They work best when the nap is kept brief so I don’t enter deep sleep and wake up groggy. They’re handy for studying, shift work, or on long drives, but they’re not a good idea too close to bedtime because the caffeine can disrupt nighttime sleep.
Most people should avoid drinking caffeinated coffee about 6 hours before bedtime, because caffeine can stay in your system for a long time. For many adults, caffeine has a half-life of roughly 5–6 hours, which means that even one cup in the late afternoon can make it harder to fall asleep or reduce sleep quality. If you’re sensitive to caffeine, you may need even more time—sometimes 8 hours or more. If you tolerate caffeine well, you might get away with a shorter window, but stopping 6 hours before bed is a good general guideline.
I usually avoid caffeine after midday for this reason, but once in a while I will take advantage of the coffee nap hack to boost my alertness, especially if I am planning on driving. I have found that soon after lunch I start to feel listless, and that's the time to take a coffee nap.
I note that this also works (with somewhat reduced efficacy) using tea. A strong cuppa followed by a wee nap worked wonders for my grandma, who often applied this tactic at home. one moment listless and crabby, but after her nap, energetic and raring to go.