When in a sleep-deprived state, there is a balance of sleep pressures called a pressure ratio. This is a ratio of REM pressure vs SWS pressure, and as your body recovers sleep debt throughout our adaptation, the pressure ratio will change. A high ratio means REM pressure is greater than SWS, and low pressure means SWS pressure is greater than REM pressure.
Glial Fatigue in General
Sleep pressure is caused by glial fatigue, which is greatly increased by CNS activity and learning processes. Sleep pressure increases when glial fatigue causes regulation of a sleep-inducing neurotransmitter in the brain called adenosine.
Part of glial fatigue is when pro-activity neurotransmitters are used up in the brain faster than can be replaced, and as the brain becomes deplete of neurotransmitters such as acetylcholine, dopamine, and glutamine. Another part of glial fatigue is when ATP is broken down for fuel and free adenosine acts as a signal to A1 receptors and A2A which increases Sleep Pressure.
REM pressure increases with greater adenosine reception. Caffeine blocks adenosine receptors, stopping the accumulation of adenosine being detected, which hinders recovery, as REM sleep is then reduced. When there is a high enough REM Pressure, the body will lower the brain to an alpha wave frequency state in preparation for REM sleep.
The process of REM sleep recovery involves two main actions. Norepinephrine, serotonin and histamine secretion is shut down so that monoamine receptors can re-sensitize. Many parts of the brain are neurally dissociated. GABA, dopamine and acetylcholine shuttle to the Basal Ganglia and GABA and glutamine shuttle to the Cerebellum for sorting of procedural memories. The similar combinations of neurotransmitters shuttle to the hippocampus, parietal cortex, entorhinal cortex, prefrontal cortex, retrosplenial cortex, perirhinal cortex mostly working on NMDA receptors for sorting of spatial memory.
SWS pressure increases with a drop in membrane potential bistability in Oligodendrocytes and Satellite Cells. Calcium signaling is how glial cells integrate and propagate signals in the central nervous system.
SWS pressure also increases because there is an accumulation of oxidative damages from free radicals created when Astrocytes create ATP through oxphos. ‘Junk’ accumulates inside and in between the brain cells which need to be cleaned out, and mutation damage occurs which needs to be fixed.
SWS pressure is regulated by sleep spindle occurrences and periodic lowering of high-frequency brain activity, and in simple terms, the slowing of brain activity is a result of nervous exhaustion (reduction in calcium uptake following chronic depolarization) and oxidative damage. When there is a high enough SWS Pressure, the body will purposely lower the brain to a delta wave frequency state in preparation for SWS.
There is a certain rate at which cells can transfer neurotransmitters and nutrients, and when a brain enters into SWS the rate of use of neurotransmitters and nutrients drops while the rate of transfer of nutrients stays the same or increases so the cells refill with antioxidants and neurotransmitters faster than they use them. Schwann Cells clear cellular debris that allows for regrowth of PNS neurons. As SWS continues, serotonin and noradrenaline are shuttled to the amygdala, the hippocampus, the rhinal cortex, the thalamus and the prefrontal cortex for consolidation of declarative memories.
Adenosine breakdown is fast during REM (and can clear in 20 minutes) so this system explains why high-frequency REM sleep can be so much more refreshing than large blocks of REM as experienced by a monophasic sleeper.
In healthy individuals, all SWS happens in the first 3h of a night’s sleep so this explains why an individual only needs 3-3.5h uninterrupted core sleep for complete memory consolidation.
The first few days of sleep deprivation both REM and SWS pressure will rise, but the REM: SWS pressure ratio will be high while REM pressure builds faster. As days pass and REM sleep rebounds in naps, the ratio will equalize and there may be a lulling point. Very often, however, a person’s REM rebound will be so powerful that the ratio drops low and soon afterward there is an SWS rebound.