A literature review by Mitchell & Phillips (2007) examined how positive and negative moods influence people’s cognitive functioning, or more specifically, executive functions.
Positive moods impair certain aspects of people’s executive functioning. The increased dopamine levels found in happy people are probably responsible for this effect on cognition, the authors say. For this reason, a positive mood seems to impair analytic thinking skills.
But when a task is novel then positive moods actually improve cognitive flexibility, i.e. the ability to switch between search or solution strategies. So, a positive mood helps people solve problems fast and efficiently via so-called heuristics.
A negative mood seems to have only minor effects on executive functions. Negative moods seem to facilitate systematic and analytic thinking. So, a negative mood may facilitate long-term planning processes and reasoning.
In the case of negative moods, serotonin seems to play an important role in executive functioning, but the intensity and duration of negative mood, and the excess of serotonin, is probably just as important as the negative mood itself, but this is not something earlier studies have looked at.
It’s not clear which neurotransmitters are involved in the brain’s executive functioning, but dopamine and serotonin is probably just a few, and these should not be considered as isolated quantities, but as interacting.