A lot of people just don't feel quite human without that morning cup of coffee. Now a study finds that the enhanced sense of well-being that caffeine can cause is reflected in our perception of words. Specifically, caffeine increases the ability to recognize words associated with positive thoughts, but doesn't provide the same boost for words with negative or even neutral associations. The research is in the journal PLoS One.

Scientists assigned 66 subjects to one of two groups. Half got a 200-milligram caffeine tablet, a dose equal to almost three cups of coffee. The other half received a sugar tablet. Thirty minutes later the volunteers were shown strings of letters, and had to decide as fast as they could if a string formed a word or was just gibberish.