dopamine is a "feel good hormone," and is normally released when you feel pleasure, or are happy.
a lot of stimulants also act on dopamine and raise it to a level much higher then what is natural, by altering the way that your dopamine receptors act. basically you end up with an unregulated dopamine dump thing going on in your brain to some degree. Methamphetamine and cocaine will do this.
It is widely known that nicotine is a stimulant, and that it triggers dopamine release. this correlation may be what causes the misconception that dopamine itself is a stimulant, when it is not.
Dopamine release just helps you exist in a more pleasent mood, which is relaxing in itself, and thus promoting sleep.