18.12: Sleep-Wake Cycles
Sleep is an essential physiological process vital to maintaining overall well-being. The reticular activating system (RAS), a network of neurons in the brainstem, regulates wakefulness and sleep. While it may seem passive, sleep consists of distinct cycles, each with its unique characteristics and functions. Two key sleep phases are non-rapid eye movement (NREM) and rapid eye movement (REM).
NREM Sleep
NREM sleep comprises four progressive stages that seamlessly merge:
- Stage 1 is the transitional phase bridging wakefulness and sleep. This stage usually lasts between 1 and 7 minutes, during which the person feels relaxed and has closed eyes. Individuals awakened during this stage often express a sense of not having been asleep.
- Stage 2, characterized as light sleep, marks the onset of the sleeping state. It is relatively easy to rouse someone during this stage. Fragmented dreams may occur, and the eyes may exhibit slow horizontal movements.
- Stage 3 denotes a period of moderately deep sleep. The individual experiences decreased body temperature and blood pressure, making it slightly more challenging to awaken them. Typically, Stage 3 emerges about 20 minutes after falling asleep.
- Stage 4 signifies the deepest level of sleep. Although brain metabolism considerably decreases and there is a slight drop in body temperature, most reflexes remain intact, and muscle tone only undergoes minimal reduction. Waking an individual during this stage becomes exceedingly difficult.
REM Sleep
REM sleep is an intriguing phase characterized by rapid eye movements and intense brain activity including vivid dreams. While the body appears still and relaxed, internally, the brain is highly active. The muscles are temporarily paralyzed to prevent acting out dreams.
Sleep Disorders
Sleep-related health issues are a serious concern and can manifest in various forms, including insomnia, sleep apnea, and narcolepsy. Insomnia is characterized by persistent difficulty in falling or maintaining sleep, often as a consequence of stress, overconsumption of caffeine, disrupted sleep-wake cycles, or depression. Sleep apnea, in contrast, is a condition where the sufferer's breathing involuntarily stops for periods exceeding 10 seconds during sleep, usually due to the throat's muscular tone diminishing, causing the airway to constrict. Narcolepsy is unique in that it is not associated with an inability to sleep but rather a failure to stay awake. Sufferers experience sudden bouts of sleep during their waking hours. Research has linked this to a lack of the neuropeptide orexin, also known as hypocretin, which aids in maintaining wakefulness. A malfunction in the hypothalamus, where orexin is produced, can be a critical factor in the development of narcolepsy.
