Only two electrons can orbit in a particular energy level. This is called the Pauli Exclusion Principle. Electrons also rotate as they orbit. The two electrons in an energy level will usually rotate in opposite directions (singlet state), though they also might rotate in the same direction (triplet state). In fluorescence, the two excited electrons are in the singlet state, while in phosphorescence, they are in the triplet state. Phosphorescence emits light longer because it takes more time for the electrons to fall to their former singlet ground state.