14.3 Circadian rhythms and sleep physiology

Your body's internal clock is like a conductor, orchestrating when you sleep, eat, and function. The suprachiasmatic nucleus in your brain is the maestro, keeping everything in sync with the day-night cycle.

Sleep isn't just rest—it's a complex process vital for health. Different stages of sleep serve unique purposes, from memory consolidation to hormone regulation. Messing with your sleep can throw your whole system out of whack.

Suprachiasmatic Nucleus in Circadian Rhythms

Role of the Suprachiasmatic Nucleus (SCN)

• Acts as the master circadian pacemaker, regulating the body's internal 24-hour clock
• Small region in the hypothalamus (located in the brain)
• Receives light input from the retina via the retinohypothalamic tract
  • Allows synchronization of the circadian rhythm with the external light-dark cycle
• Sends signals to other brain regions and peripheral organs to coordinate their circadian rhythms
  • Ensures physiological processes occur at the appropriate times of day (hormone secretion, sleep-wake cycle)

Consequences of SCN Damage

• Disrupts circadian rhythms, leading to various health issues
  • Sleep disorders (insomnia, excessive daytime sleepiness)
  • Metabolic dysfunction (impaired glucose metabolism, obesity)
  • Mood disorders (depression, anxiety)
  • Weakened immune function

Sleep-Wake Cycle Physiology

Regulation of the Sleep-Wake Cycle

• Involves the interaction of two processes: circadian process and homeostatic process
  • Circadian process is driven by the SCN and follows a roughly 24-hour cycle
  • Homeostatic process builds up sleep pressure during wakefulness
• Influenced by hormones and neurotransmitters
  • Melatonin, produced by the pineal gland, promotes sleep (reduces alertness, lowers body temperature)
  • Adenosine accumulates in the brain during wakefulness, increasing sleep pressure
  • Cortisol peaks in the morning to promote wakefulness
  • Growth hormone is released during deep NREM sleep

Physiological Changes During Sleep Stages

• Non-rapid eye movement (NREM) sleep
  • Reduced heart rate, blood pressure, and muscle tone
  • Decreased brain activity and metabolism
• Rapid eye movement (REM) sleep
  • Increased brain activity and vivid dreaming
  • Muscle atonia prevents physical movement during dreams
  • Plays a role in memory consolidation and emotional processing

Impact of Sleep Deprivation and Disruption

Cognitive and Emotional Consequences

• Impairs cognitive function, including attention, memory, and decision-making abilities
• Affects emotional regulation, leading to increased irritability and mood swings
• Increases the risk of depression and anxiety disorders

Metabolic and Immune System Effects

• Associated with an increased risk of obesity and diabetes
  • Alters appetite-regulating hormones (leptin and ghrelin)
  • Impairs glucose metabolism and insulin sensitivity
• Weakens immune function, making individuals more susceptible to infections
• Increases the risk of cardiovascular disease (hypertension, stroke)

Consequences of Circadian Disruption

• Experienced by shift workers or during jet lag
• Leads to sleep disturbances and gastrointestinal issues
• Increases the risk of certain cancers (breast, prostate, colon)
• Causes misalignment of hormone secretion, leading to metabolic dysfunction

Mechanisms and Consequences of Sleep Disorders

Insomnia

• Difficulty falling asleep or staying asleep
• Caused by stress, anxiety, depression, or poor sleep hygiene
• Leads to daytime fatigue and impaired functioning

Sleep Apnea

• Breathing repeatedly stops and starts during sleep
• Causes fragmented sleep and excessive daytime sleepiness
• Increases the risk of cardiovascular disease (hypertension, heart failure)

Restless Leg Syndrome (RLS)

• Neurological disorder causing an irresistible urge to move the legs
• Often accompanied by uncomfortable sensations
• Disrupts sleep and leads to daytime fatigue

Narcolepsy

• Chronic sleep disorder characterized by excessive daytime sleepiness and sudden sleep attacks
• Often accompanied by cataplexy (sudden loss of muscle tone) and sleep paralysis
• Caused by a deficiency in the neurotransmitter hypocretin (orexin)

Parasomnias

• Abnormal behaviors or experiences during sleep
  • Sleepwalking, night terrors, and REM sleep behavior disorder
• Can be disruptive to both the individual and their sleep partners
• May be triggered by stress, sleep deprivation, or certain medications

Circadian Rhythm Sleep Disorders

• Misalignment between the individual's sleep-wake cycle and societal norms
  • Delayed sleep phase disorder (difficulty falling asleep and waking up at conventional times)
  • Advanced sleep phase disorder (falling asleep and waking up earlier than desired)
• Leads to difficulty functioning in daily life (work, school, social activities)
• Can be managed with light therapy, melatonin supplementation, and behavioral interventions