Does being tired make it hard to be happy?

Via NurtureShock:

Negative stimuli get processed by the amygdala; positive or neutral memories gets processed by the hippocampus. Sleep deprivation hits the hippocampus harder than the amygdala. The result is that sleep-deprived people fail to recall pleasant memories, yet recall gloomy memories just fine. In one experiment by Walker, sleep-deprived college students tried to memorize a list of words. They could remember 81% of the words with a negative connotation, like âcancer.â But they could remember only 31% of the words with a positive or neutral connotation, like âsunshineâ or âbasket.â