Wavelet transform and FFT using to extract feature power bands with EEG signals
Alpha rhythm
Generally, alpha-band oscillatory activations (8-10Hz) relate to relaxation and in principle accompanied with closure of eyes. This is the prime marker that is used to detect drowsiness, but surely not the only one (see alpha dropout, NREM1, eye-rolling EEG artefacts).
Event detection
To detect alpha, electrodes from occipital or adjacent regions are used. Due to low SNR in EEG signals and its variance across EEG systems, you may want to try different detection methods to decide for your classifier.