Caffeine has always caused me to have a heightened level of anxiety that lasts much longer than initial high. My recommendation is that you should sleep when you are tired. I like to take a nap in my car during lunch.
Unrelated, but: http://theoatmeal.com/comics/caffeine
For me, coffee is drastically inferior to the high I get from earl grey tea (most black teas in general). I find that with tea the high is much smoother and lasts longer; if you can imagine a bell curve, it would be wider and with a smaller amplitude, whereas coffee would be narrower and with a taller amplitude.
Also, I've found a 20 minute nap at mid-day with green tea afterwards (about half the caffeine content of my morning breakfast tea) to be much more effective in improving cognitive performance than to skip the nap and rely solely on caffeine. For the rest of the day I'm caffeine free.
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