One of my neuroscience teachers explained to us how wonderful Modafinil is: it keeps you awake and fully concentrated for hours and hours, without apparent side effects. Modafinil is ostensively a drug prescribed for narcolepsy, but it’s clear that many people use it simply to stay awake and work. Given how happy he sounded about it, I couldn’t help but think that my teacher must have used it himself.
I looked into it too. The only ways I could find to get Modafinil without a prescription were too expensive for a graduate student (which I was at the time). So I gave up and never even tried it 
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Now Nature has just published a commentary on the use of cognitive-enhancing drugs by academics. It also has a forum where you can read and write comments on this topic.