Chapter 14: Biological Rhythms, Sleep & Dreaming

Biological Rhythms, Sleep & Dreaming begins by defining circadian rhythms, which are endogenous cycles lasting approximately 24 hours, distinguishing between diurnal and nocturnal patterns. The central pacemaker for these rhythms in mammals is identified as the suprachiasmatic nucleus (SCN) of the hypothalamus. The text details the molecular clockwork driving the SCN, involving a feedback loop of specific proteins—Clock, Cycle, Period, and Cryptochrome—whose transcription and degradation rates determine the circadian period. The process of entrainment is explained, wherein zeitgebers like light synchronize the internal clock to the solar day via the retinohypothalamic pathway, utilizing specialized retinal ganglion cells containing the photopigment melanopsin. The chapter then shifts to the architecture of human sleep, characterizing the electroencephalography (EEG) patterns associated with different states: the desynchronized beta activity of wakefulness, the alpha rhythms of relaxation, and the progression through non-REM (NREM) stages 1, 2, and 3 (slow-wave sleep), characterized by vertex spikes, sleep spindles, K complexes, and high-amplitude delta waves, respectively. It contrasts these with Rapid Eye Movement (REM) sleep, or paradoxical sleep, which features an awake-like EEG and muscle atonia. Changes in sleep patterns across the lifespan are examined, noting the high prevalence of REM in infancy and the significant reduction of slow-wave sleep in the elderly. The biological functions of sleep are categorized into four main theories: energy conservation, ecological niche adaptation, body restoration (including immune function and glial waste clearance), and memory consolidation. The neurological control of sleep is attributed to four interacting systems: a forebrain system generating SWS, a brainstem reticular formation promoting wakefulness, a pontine system (subcoeruleus) triggering REM and muscle paralysis, and a hypothalamic system utilizing the neuropeptide hypocretin (orexin) to coordinate switching between states. The chapter concludes by reviewing sleep disorders, particularly narcolepsy—caused by the loss of hypocretin neurons—as well as sleep apnea, REM behavior disorder, insomnia, and parasomnias like night terrors and sleepwalking, while also touching upon the limitations of pharmacological interventions.