The main causes of highways mortality are fatigue and sleepiness while driving. In this paper, we propose a method to detect the transition between wakefulness and sleepiness states based on driver's brain activity recorded through Electroencephalography (EEG). Our new method uses the Phase Locking Value (PLV)to extract data that are directly related and correlated to the sleepiness. PLV measures changes in the neuronal function. Results show the performance of PLV and its ability to improve the performance of already existing systems.