When you’re low on sleep, it all looks good: I get a bit cranky when I’m not getting a sufficient amount of sleep, but I have seen people who get a bit giddy instead. This latter reaction is apparently the norm. After a bout of sleep deprivation, a trip to the MRI shows that subjects tend to have an overactive reward system, making them more prone to judge emotional stimuli as pleasant compared to control subjects. The risk here is that anyone trying to make a decision after pulling an all-nighter will be prone to assuming everything will work out well.