There is growing interest in sleep research and finding easily tracked neural correlates of brain states is a central challenge to the definition of sleep and wake states. Here we demonstrate using multi-site electrophysiological LFP recordings in freely moving mice that gamma power in the olfactory bulb (OB) allows for clear classification of sleep and wake. Coupled with hippocampal theta activity, it allows the construction of a robust and reproducible sleep scoring algorithm that relies on brain activity alone. We validate the procedure by comparison with classical methods based on muscular activity (EMG) and video tracking. Contrary to EMG, OB gamma power allows correct discrimination between sleep and immobility in ambiguous situations such as fear-related freezing. Finally, beta power in the OB is a good predictor of Rapid Eye Movement sleep. Overall, our results reveal the OB can be used as a highly reliable readout of brain states.