Surprisingly little is known about the effects of sleep deprivation on affective processes. Although clinical evidence and introspection suggest that emotional function is sensitive to sleep loss, there are only three published studies that have experimentally manipulated both stress and emotion in a single experiment, the earliest of which was published in 2007. This dissertation presents findings from three studies that were designed to improve our understanding of the influence of sleep loss on affective functioning in healthy adults. Study 1 (Sleep and Mood) measured the effects of sleep loss on affect in the absence of specific probes. Three facets of mood (Fatigue, Vigor and Confusion) were found to be sensitive to sleep restriction, increasing in a dose-response manner with extended wakefulness and covarying with a well validated behavioral assay of alertness (the PVT reaction time task). Three other facets of mood (Depression, Anxiety, and Anger) were not sensitive to sleep restriction and did not covary with objective alertness. Study 2 (Sleep and Emotion) found that sleep deprivation decreased facial expressiveness in response to positive and negative emotion probes. There was also a trend toward decreased intensity of positive and negative subjective emotional reactions for sleep deprived subjects as well. Study 3 (Sleep and Stress) found that sleep deprived subjects reported a more negative subjective response than control subjects to a mild stressor, but not to a more intense stressor. When taken together, these studies offer a more nuanced account of the relationship between sleep deprivation and affective functioning. This dissertation ends with a discussion of the implications of these findings for both healthy and clinical populations and proposes future direction for research on sleep and emotion.