Background: Recently there has been a revitalization in research on classic psychedelic substances. This class of drugs has been found to produce intense and profoundly meaningful experiences, and offers a unique opportunity to study the neural correlates of the sense of self. The objective of this research was to systematically review the effects of classic psychedelics on spontaneous brain activity, as measured on three electrophysiological modalities: spectral analysis, signal diversity, and functional connectivity. Method: We searched Pubmed to identify papers in English, published between January 1990 to May 2021, where electrophysiological methods were used to evaluate the effects of classic psychedelics in healthy individuals during non-task resting states. Results: Sixteen papers were included. Classic psychedelic substances generally decrease spectral power in most frequency bands, mainly in the alpha range, increase signal diversity, and decrease the flow of information throughout the brain. Conclusion: Decreases in alpha power, increased signal diversity, and decreases in default mode network activity might be important neural correlates of the psychedelic state. However, inconsistencies in the results and heterogeneity in study design are some of the limitations that have to be considered when interpreting these results.