Human restorative sleep consists of repeated cycling of brain wave activity from light sleep (NREM1) to moderate sleep (NREM2) to deep, slow wave sleep (SWS, or NREM3), to REM sleep. The hypnogram image on this page demonstrates the cycling through NREM and REM stages throughout the night. Each cycle lasts about 90 minutes.
These continuous cycles, when they are not interrupted, is what allows the brain and body to heal across the night. For SWS to occur, the Reticular Activation System (RAS) has to quiet down. This can only happen in the absence of outside threats. The SWS then creates the conditions necessary for basic brain and body repair. For example, memories are transferred to long term storage, cell energies are restored, nutrition stored in cells, and damaged cells and proteins cleared. This is what is needed to provide a deeply restful sleep.
The NREM stages are followed by REM Sleep, a period of sleep in which left and right hemispheres are actively communicating. REM primarily benefits integrative processes like emotional regulation, creative problem-solving, and cognitive flexibility. Here is a summary of how each of these stages accomplish preparing your body and mind for the next day.
Autonomic Nervous System Regulation: NREM2 supports cardiovascular stabilization, to prepare for deeper autonomic recovery during NREM3.
Immune Function: NREM2 plays a role in consolidation of immunological memories, enhancing pathogen recognition and response, providing you with better protection from illness.
Emotional Regulation: NREM2 activates emotionally relevant memories from the previous day, feeding the most relevant daily experiences to NREM3. This supports emotional resilience.
Cognitive Functions: NREM2 consolidates procedural and motor memory through sleep spindles (bursts of brain activity). This facilitates learning through hippocampal to neocortical transfer of declarative (fact-based) memories.
Autonomic Nervous System Regulation: The Reticular Activating System (RAS) shuts down to allow the slow delta waves of NREM3. The delta waves reflect the synchronization of cortex with hippocampus. During synchronization memories transfer from short-term to long-term (neocortex) memory. Reduced RAS activity is reflected in reduced sympathetic activity, which promotes parasympathetic dominance, aiding cardiovascular recovery. Both blood pressure and heart rate are stabilized at night and reduce daytime reactivity.
Hormonal Regulation: Shifts from sympathetic to parasympathetic dominance further suppresses cortisol during the early part of the night, promoting a state of recovery. The first NREM3 stage stimulates growth hormone release for tissue repair and muscle recovery. NREM3 stages conserve energy by lowering metabolic rate and glucose utilization.
Immune Function: NREM3 boosts immune cell production, including T-cells and natural killer (NK) cells. SWS also enhances cytokine release, supporting the immune response to infections and inflammation.
Cognitive Functions: NREM3 reinforces synaptic plasticity and clears metabolic waste from the brain via the glymphatic system. These processes support optimal brain function next day and reduce the risk for dementia in the longer term.
Autonomic Nervous System Regulation: REM improves heart rate variability, promoting stress resilience and autonomic balance.
Hormonal Regulation: REM balances the hypothalamic-pituitary-adrenal (HPA) axis to maintain normal cortisol rhythms. REM supports cerebral metabolic recalibration with increased blood flow and oxygenation to the brain.
Emotional Regulation: REM facilitates emotional processing and resolution of negative experiences through vivid dreaming. REM reduces amygdala reactivity, decreasing fear and anxiety responses and supporting greater emotional stability.
Cognitive Functions: REM enhances creative problem-solving and cognitive flexibility by integrating disparate information. REM strengthens emotional memories and promotes learning through synaptic pruning and reinforcement. REM supports the integration of new experiences into autobiographical memory, which supports better future planning.