This chapter provides an overview of the chief epigenetic mechanisms: DNA cytosine methylation and hydroxymethylation, histone protein post-translational modifications (PTMs) and higher-order chromatin remodeling, non-coding RNA (ncRNA) regulation, and RNA editing. High level of 5-methylcytosine (5mC) is found in gene regulatory regions and generally associated with transcriptional repression. This occurs because DNA methylation inhibits the transcriptional machinery from accessing DNA. Chromatin is implicated in a broad range of processes including transcriptional regulation, X chromosome inactivation (XCI), genomic imprinting, DNA replication and repair, and the maintenance of genomic integrity. RNA editing events can alter amino acids encoded by corresponding genomic DNA, alternative splicing patterns, and regulatory sequences and their associated interactions. Preliminary data suggest that DNA methylation is modulated by circadian rhythms. DNA methylation at specific genomic loci can play a role in the pathophysiology of sleep disorders.