Researchers gave 28 participants two different sleep aids--zolpidem (Ambien), which has been shown to increase the density of sleep spindles, and sodium oxybate (Xyrem) which decreases spindle activity. They then viewed one-second images designed to provoke a positive or negative response, before taking supervised naps. (Science: Just like kindergarten!) After taking zolpidem, their memory was enhanced--but it only increased their ability to remember the negative images, specifically those that were considered "high arousal" (like pictures of a hairy car wreck or a snake poised to attack, rather than an image of people standing around a grave). Somewhat surprisingly, sodium oxybate did not lead to a decrease in the ability to recall negative emotions, despite the decrease in spindle activity.