What are the fundamental changes in the nervous system in NREM versus REM sleep, and what other differences are noted between the phases of NREM and REM?
Cortical deactivation and autonomic nervous system (ANS) changes typify sleep. While awake, we interact with our environment, but when asleep we are largely unresponsive to sensory input and do not generate motor output. During NREM, the cortex is deactivated, EEG activity is slower and of higher voltage than when awake, and blood flow is decreased. During REM sleep, the cortex is activated, EEG readings during sleep are similar to those when awake, and blood flow is increased. There are also changes in the ANS during sleep, with parasympathetic nervous system (PNS) predominance in NREM and even more so in REM. Sympathetic nervous system (SNS) tone decreases in NREM and usually in REM, but PNS tone and SNS tone can be variable during REM. In NREM, there are decreases in respiratory rate (RR), heart rate (HR), blood pressure (BP), and cardiac output. Normal REM is characterized by fluctuations in BP, HR, and RR. Dreaming and somatic muscle hypotonia to atonia (which includes reduced to absent upper airway muscle tone) are also characteristic REM events. REM may have a few periods of decreased or absent breathing. Cerebral metabolic rates for glucose and oxygen decrease during NREM, but increase to above waking levels in REM. As mentioned, REM-predominant OSA is associated with pre-DM and T2DM.
Comparison of Sleep Stages
|Responsiveness to stimuli||Reduced||Reduced to absent|
|Sympathetic activity||Reduced||Reduced or variable|
|Parasympathetic activity||Increased||Markedly increased|
|Respiratory rate||Decreased||Variable; apneas can occur|
|Muscle tone||Reduced||Markedly decreased|
|Upper airway muscle tone||Reduced||Moderately decreased to absent|
|Cerebral blood flow||Reduced||Markedly increased|
|Other characteristics||Sleep walk||Dreams|
REMs, Rapid eye movements; SEMs, slow eye movements.
Modified from: Chokroverty, S. (2006). Disorders of sleep. In Neurology (Chapter XIII). American College of Physicians Medicine, WebMD Inc.