We have studied electron spin dynamics in self-organized InAs/GaAs quantum dots (QD's) by time-resolved photoluminescence (PL). The electron polarization of the QD discrete energy levels is determined from the circular polarization of the QD emission. When the GaAs barrier is optically pumped, a transient increase of the QD excited-state PL polarization is observed. This effect results from the spin dependence of electron energy relaxation as a consequence of the Pauli exclusion principle. A theoretical model qualitatively describing the experimental results is developed.