The very idea of sleep EEG-based brain age index (BAI) comes from the observation that sleep changes with age. In the literature, there are many studies describing various aspects of sleep changing with age. However, the descriptions are scattered. There is a critical gap that age can be predicted by summarizing these changes in a systematic manner. Since the model is based on relatively healthy subjects, the deviation of the predicted “brain age” (BA) should reflect accelerated or slowed aging when compared to the chronological age (CA). We then validated BAI for various outcomes including brain aging, cognition, neurodegeneration, neuro-infectious diseases, and neurological disorders. Overall, the results show that BAI is associated with these outcomes. We also investigated the night-to-night variability of BAI since it is a particularly important concern for sleep-based biomarkers.