Over the last decades, Polysomnography (PSG) has been the most widely used clinical measure of sleep where sleep is described as NREM sleep and REM sleep based on Electroencephalogram (EEG) morphology. NREM sleep is presented in 3 sleep stages and REM sleep is also often referred to as dream sleep.
SleepImage, Cardiopulmonary Coupling (CPC) is based on the physiological changes in the autonomic nervous system that occur during sleep. It integrates information from the brain electrical activity through the autonomic nervous system. Analyzing heart rate variability (HRV) coupled with respiration, CPC captures the essence of sleep by looking at the ebb and flow of slow wave power that is the accepted marker of sleep drive in humans and in non-human species. The CPC-method does not rely on the same data streams as PSG and the output is not meant to match PSG, it however complements conventional sleep staging, albeit with a different method of categorizing sleep. Rather than being dependent on manual interpretation, primarily of EEG morphology, the automated output reveals that NREM sleep has a distinct bimodal-type structure marked by distinct alternating and abruptly varying periods of high and low frequency CPC-power. High frequency coupling (HFC) or stable sleep occurs during stage part of NREM-2 and all NREM-3 and is associated with periods of stable breathing, non-cyclic alternating pattern (n-CAP) EEG, increased absolute and relative delta power, strong sinus arrhythmia and blood pressure dipping. Conversely, low frequency coupling (LFC) or unstable sleep occurs during NREM-1 and part of NREM-2 and has opposite features and is characterized by temporal variability of tidal volumes, cyclic alternating pattern (CAP) EEG and non-dipping of blood pressure, lower frequency cyclic variation in heart rate. CPC defines REM sleep into Stable and Unstable REM sleep based on frequency analysis of how the dominant CPC state has been classified as vLFC, where fragmented REM sleep is often accompanied by elevated Low Frequency Coupling.
In a healthy sleep pattern, cycles between Stable, Unstable and REM sleep (NREM Stage 1, 2 and 3 & REM sleep cycles on PSG) occur every 30 – 90 minutes and approximately 4-8 cycles occur during an 8 hour healthy sleep period. The ratio of NREM sleep to REM sleep in each cycle varies during the course of the sleep period. The first episode of REM sleep may last only a few minutes, but time periods spent in REM sleep increase progressively over the sleep period, with the final period of REM sleep that may last up to 30 minutes. In summary, Stable NREM sleep (slow-wave sleep) is prominent in the first third of the night and REM sleep is prominent in the last third of the night.
Stage 2 NREM sleep = Unstable & Stable Sleep – accounts for 45-55% of total sleep time. This is the first stage of effective sleep and each period lasts about 10-25 minutes. The characteristic EEG findings of this stage are sleep spindles believed to occur when the brain disconnects from outside sensory input and begins the process of memory consolidation and K complexes that are sort of built-in vigilance system that keep you poised to awake if necessary. Delta waves first appear during this period of sleep but are present in small amounts. Most people spend about half of the night in this stage, where eyes are still, and heart rate & breathing gradually slows down.
Stage 3 NREM sleep = Stable Sleep – accounts for 15-20% of the total sleep period. The characteristic EEG findings of this stage are that slow-brainwaves or Delta waves become dominant. The brain becomes less responsive to external stimuli, making it difficult to wake up the sleeper. Slow-wave sleep is the time for the body to renew and repair. During this sleep stage muscle tone decreases, breathing becomes more regular, blood pressure drops, and pulse rate slows. Blood flow is directed less toward the brain and at the beginning of this stage the pituitary gland releases a pulse of growth hormone (GH) that stimulates tissue growth. When a sleep deprived person gets some sleep, he or she will pass quickly through the lighter sleep stages, into the deeper sleep stages and spend a greater proportion of the sleep period in this stage. This is believed to indicate that slow-wave sleep has an essential role in a person’s optimal functioning.