Core sleep is the essential portion of the sleep cycle that includes the most restorative stages, primarily deep sleep (Stage 3 of NREM) and REM sleep. It is critical for physical repair, cognitive function, memory consolidation, emotional processing, and overall well-being. Core sleep typically occurs during the first half of the night and usually makes up about 35-45% of total sleep time, or roughly 2.5 to 4 hours for most adults. It sets the foundation for sustained physical and mental energy throughout the day, and insufficient core sleep can negatively affect memory, concentration, immune function, and overall health. Deep sleep in this core phase supports tissue repair, immune strengthening, and rejuvenation, while REM sleep supports brain activity for learning and emotional regulation.