Pathophysiology of Sleep Apnoea
Sleep-Wake Regulation
The interaction between sleep-wake regulation and the control of breathing has been implicated in the development of sleep apnoea.
Sleep-wake regulation is the biological process that governs the balance between sleep and wakefulness in humans and other animals. It involves a complex interaction between multiple brain regions, neurotransmitters, and hormones to maintain a stable sleep pattern and ensure an adequate amount of sleep is obtained. Sleep-wake regulation is primarily controlled by two interacting systems:
1. The circadian rhythm: This is the body's internal 24-hour clock that regulates various physiological processes, including the sleep-wake cycle. The circadian rhythm is regulated by the hypothalamus, which synchronizes with external light-dark cycles and other time cues.
2. Sleep homeostasis: This is the process by which the body maintains a balance between sleep and wakefulness based on the amount of time spent awake. The longer an individual is awake, the stronger the drive to sleep becomes, and vice versa. Sleep homeostasis is influenced by the accumulation of sleep-inducing substances in the brain, such as adenosine, which build up during wakefulness and decrease during sleep.
The interplay between these two systems determines the timing, duration, and quality of sleep.
Sleep is a dynamic process with rapid transitions between non-rapid eye movement (NREM) and rapid eye movement (REM) sleep stages.
Breathing control is modulated during these transitions with REM sleep being particularly vulnerable to the development of apnoeas due to decreased muscle tone and increased upper airway collapsibility.
Additionally, the hypercapnic (high CO2) and hypoxic (low O2) ventilatory responses are reduced during sleep, especially in REM sleep, which can exacerbate the severity of sleep apnoea.