3.10: Sleep

Sleep is a recurring physiological state marked by altered consciousness, diminished sensory activity, and the suppression of nearly all voluntary muscular actions. It is differentiated from wakefulness by reduced responsiveness to stimuli and possesses a higher degree of reversibility than a comatose state.

Although the exact reason for sleep remains unclear, experts speculate that sleep provides neurons with chances for rest and regeneration. Without sleep, neurons might succumb to energy depletion or accumulation of waste products from cellular processes, culminating in malfunctions. Moreover, sleep affords the brain a platform for reinforcing crucial neuronal connections that may otherwise deteriorate due to inactivity. Concurrently, the release of growth hormone in children and young adults coincides with the deep sleep phase. This sleep phase also stimulates heightened synthesis and diminished degradation of proteins in numerous bodily cells. Notably, neural regions governing emotions, decision-making, and social interactions exhibit substantial quiescence during deep sleep. This observation implies that a specific sleep category contributes to preserving optimal emotional and social functioning during wakefulness.

Sleep-wake cycles exhibit a circadian rhythm with an approximate 24-hour periodicity. During the sleep phase, most bodily systems shift to an augmented anabolic state, amplifying growth and rejuvenation processes across multiple systems. Furthermore, sleep exerts multifaceted impacts on our daily functionality and physical and mental well-being, insights into which are still emerging. Neurons in the brainstem produce neurotransmitters like serotonin and norepinephrine that sustain the activity of specific brain regions during wakefulness. Meanwhile, distinct neurons within the brainstem initiate signaling upon sleep onset. These neurons seemingly "switch off" the signaling pathways that promote wakefulness.

The sensation of sleepiness is correlated with an accumulation of adenosine in the bloodstream while awake, leading to drowsiness. This chemical substance gradually disintegrates during the sleep phase. Caffeine consumption operates by inhibiting adenosine receptors in the brain, effectively counteracting the suppressive effect of adenosine on the arousal center.

The electroencephalogram (EEG) is obtained by placing multiple electrodes on specific cranial regions, capturing cumulative responses of excitatory and inhibitory postsynaptic potentials originating in the cerebral cortex. Alpha waves manifest at 8 to 13 Hz frequency, are observed during wakefulness and rest, and are absent during sleep. Beta waves emerge during CNS activation, operating at 14 to 50 Hz. Theta waves occur during emotional stress in some adults and are more frequent in children, ranging from 4 to 7 Hz. Delta waves encompass all frequencies below 3.5 Hz and are predominant during sleep. Gamma waves are a type of wave pattern with a high frequency ranging from 25 to 100 Hz. These brain waves are associated with high level cognitive functioning.

Sleep comprises five phases: stages 1, 2, 3, and 4, and rapid eye movement (REM) sleep (Figure 26). Progressing cyclically from stage 1 to REM and restarting with stage 1, approximately 50% of total sleep occurs in stage 2, 20% in REM, and 30% in other stages. In contrast, infants spend about half their sleep time in REM sleep.

During stage 1, a light sleep phase, intermittent awakenings are possible, characterized by slow eye movements and reduced muscle activity. Fragmented visual imagery and sudden muscle contractions, often initiated by a falling sensation, are typical upon awakening from this stage. Transitioning to stage 2 halts eye movements and introduces slower brain waves interspersed with bursts of rapid sleep spindles. In stage 3, delta waves of extremely low frequency appear, interspersed with smaller, faster waves. Stage 4 is dominated by delta waves (Figure 25). Stages 3 and 4, collectively known as deep sleep, are challenging to rouse from and are marked by the absence of eye movement or muscle activity. Emerging from deep sleep leads to grogginess and disorientation lasting several minutes, accompanied by bedwetting, night terrors, or sleepwalking in children.

REM sleep is identifiable by eye movements, muscle relaxation, pupillary constriction, and penile erection. Benzodiazepine usage (depressants that cause sedation and hypnosis) and increasing age reduce REM sleep duration. The initial REM sleep period typically arises 70 to 90 minutes after onset, and a complete sleep cycle averages 90 to 110 minutes (Figure 26). Early sleep cycles feature short REM durations and extended deep sleep phases. Throughout the night, REM periods elongate, while deep sleep diminishes. Morning sleep primarily encompasses stages 1, 2, and REM sleep.⁴³

Although individuals undergoing anesthesia or existing in an unconscious state are colloquially considered asleep, their arousal is unfeasible, and their cerebral EEGs lack the intricate, dynamic patterns characteristic of typical sleep. Instead, these EEGs exhibit sluggish and feeble activity, occasionally nearly impossible. A typical EEG pattern indicative of deep anesthesia is burst suppression, characterized by alternating phases of high-voltage brain activity and periods of no brain activity. Specific characteristics may vary depending on the type of anesthetic employed. For instance, isoflurane-induced delta activity lacks the higher frequencies observed during sleep, while propofol is associated with beta oscillations. In general, volatile anesthetics appear to result in a deficit of REM sleep.⁴⁵

Insomnia, characterized by an inability to initiate or maintain sleep for the desired duration, constitutes a sleep disorder. Stress, circadian misalignment, dietary influences, and other elements can precipitate this condition.

---