Biological Basis of Sleep and Sleep Disorders

Circadian Rhythms and the Biological Clock

• Definition of Circadian Rhythm: The circadian rhythm is the body's internal biological clock that operates on a approximately $24$-hour cycle. It regulates when an individual feels sleepy and when they feel awake.
• The Average Cycle: While individual rhythms vary, the average person typically sleeps around $10:00\,PM$ and wakes up around $6:00\,AM$.
• The Urge to Sleep:
  • Sleepiness is at its highest during the actual sleep period.
  • Upon waking at $6:00\,AM$, individuals remain somewhat sleepy until approximately $8:00\,AM$.
  • The Afternoon Blip: There is a natural increase in the urge to sleep (sleepiness) between $2:00\,PM$ and $3:00\,PM$. This is often referred to as "siesta time."
  • During this blip, cognitive engagement and the ability to absorb information in lectures significantly decrease.
  • This urge naturally dissipates toward dinner time, even without stimulants like caffeine.
• Zeitgebers: Derived from the German word for "timegiver," these are environmental cues that entrain or reset the biological clock.
  • Light: The strongest and most primary zeitgeber. The presence of light promotes wakefulness, while darkness promotes sleepiness.
  • Other Zeitgebers: These include exercise and eating schedules.
• The Free-Running Cycle:
  • When human beings are deprived of light cues, their biological clock defaults to a "free-running" rhythm.
  • Research indicates that the human free-running clock is approximately $25$ hours long rather than $24$ hours.
  • Without light to entrain the cycle, individuals naturally sleep in later and stay up later each day, a pattern often observed in students during summer breaks.
• Circadian Rhythm Disruptions:
  • Jet Lag: Occurs when the internal rhythm is misaligned with the destination's time zone.
    • Westbound Travel: Easier to adjust to because it requires a "phase delay" (pulling the rhythm back/staying up later).
    • Eastbound Travel: Harder to adjust to because it requires a "phase advance" (speeding up the rhythm/trying to sleep when the body feels it is only late afternoon).
  • Shift Work: Constant movement between day and night shifts causes significant distress. Recommendations for management include:
    • Using the brightest possible lights during night shifts to trick the SCN into thinking it is daytime.
    • Using blackout blinds or tinfoil on windows to create total darkness during day-sleep.
    • Maintaining consistent eating and exercise schedules that align with the shift (e.g., eating "midday" meals at midnight).

Biological Mechanisms of Sleep Regulation

• Suprachiasmatic Nucleus (SCN):
  • A small nucleus located in the hypothalamus, specifically a bundle of cell bodies situated just below the optic chiasm (where nasal portions of the optic nerves cross).
  • The SCN contains neurons that fire on a rhythmic cycle: daytime neurons active for $12$ hours and nighttime neurons active for $12$ hours.
  • Function: It controls the cycle of sleep, not the need for sleep. Damage to the SCN results in sleeping at random intervals throughout the day and night while maintaining the same total volume of sleep.
• The Role of Melanopsin:
  • The SCN receives direct information from specialized photo-receptive ganglion cells in the retina.
  • These cells contain a specialized photopigment called melanopsin.
  • Melanopsin reacts specifically to gradual changes in light (the dawn and dusk transition) rather than sudden bursts of light.
• Melatonin Production:
  • As light levels gradually decrease, the SCN indirectly activates the pineal gland via the nervous system.
  • The pineal gland releases melatonin into the bloodstream, which induces sleepiness.
  • Exogenous Melatonin: Supplements can be used to treat circadian rhythm misalignments (e.g., in Jet Lag or Autism Spectrum Disorders) but should not be taken in the afternoon as they can disrupt the internal clock.
• Blue Light and Technology:
  • Blue light (short wavelengths) specifically activates melanopsin-containing cells, signaling the SCN that it is daytime and inhibiting melatonin release.
  • Smartphones, tablets, and computers are significant sources of blue light. Modern "night settings" shift the display from blue to yellow to mitigate this effect.
  • Aging Effects: Older individuals are less affected by blue light but more sensitive to yellow light, which can wake them up earlier.

Neuroanatomy of Sleep and Wakefulness

• The Hypothalamus: As a center for motivational behaviors (the "$4$ Fs," eating, drinking), it plays a crucial role in sleep.
  • Anterior Hypothalamus: Responsible for promoting sleep. Damage leads to permanent insomnia. Historical cases of encephalitis lethargica showed patients who could never sleep and eventually died.
  • Posterior Hypothalamus: Responsible for promoting wakefulness. Damage leads to a coma-like state or continuous sleep.
  • Interaction: The SCN inhibits the posterior hypothalamus at night to allow the anterior hypothalamus to initiate sleep. In the morning, the SCN releases that inhibition.
• The Reticular Formation (Reticular Activating System - RAS):
  • Located in the brainstem. Research on cats involved various transections to identify its role.
  • Cerveau Isolé (Isolated Forebrain): Transecting the entire brainstem puts the subject into continuous slow-wave sleep (Stage $3$ and $4$ delta waves).
  • Mid-collicular Lesion: Damaging the middle of the reticular formation specifically results in continuous slow-wave sleep, even if general sensory information can still reach the brain.
  • Stimulation: Electrically stimulating the reticular formation immediately arouses and wakes the subject.
• REM Sleep Control: Recent studies show that the reticular formation contains specific nuclei responsible for the symptoms of REM sleep:
  • Cortical EEG Desynchronization: Beta-like brain wave activity (the brain "waking up" while the body stays asleep).
  • Muscle Atonia: The inhibition of acetylcholine ($ACh$) to the body's motor systems to prevent acting out dreams.
  • Other Nuclei: Responsible for rapid eye movements, twitches in extremities, and cardiorespiratory changes.

Sleep Disorders: Insomnia and Sleep Apnea

• Insomnia: Affects approximately $25\%$ of the population regularly.
  • Types: Difficulty falling asleep (taking >$15$ minutes), difficulty staying asleep (frequent waking), or waking up too early.
  • Paradoxical Insomnia: An extreme bout of insomnia caused by the withdrawal from sleep-aid medications that the individual has become dependent upon.
  • Alcohol and Sleep: Alcohol acts as a GABA activator, helping people fall asleep, but it prevents light sleep from transitioning into Stage $3$, Stage $4$, or REM sleep. This results in poor quality sleep and grogginess the next day.
• Sleep Hygiene Recommendations:
  • Avoid stressful tasks (assignments, planning) before bed.
  • Keep the bedroom cold (body temperature naturally drops during sleep).
  • Avoid caffeine (found in coffee, soda, and chocolate) after noon.
  • Limit naps to under $20$ minutes to avoid entering slow-wave sleep.
  • Use the bedroom only for sleep to maintain a classical conditioning association between the room and relaxation.
• Sleep Apnea: A condition where an individual stops breathing while sleeping due to high carbon dioxide levels in the blood stimulating chemo-receptors to wake the person up.
  • Cause: Often due to a weak muscle flap in the throat that collapses and blocks the airway.
  • Impact: Prevents entry into deep sleep and REM, leading to chronic grogginess.
  • Treatment: Surgery to strengthen/remove the flap or the use of CPAP machines.

REM and Stage 4 Disorders

• Narcolepsy: The opposite of insomnia; characterized by "sleep attacks" where a person suddenly falls asleep for several minutes.
• Cataplexy: A symptom of narcolepsy where the individual goes directly into REM sleep, causing immediate muscle paralysis (atonia) while potentially still conscious.
• Sleep Paralysis: Occurs when a person wakes up from REM sleep, but the reticular formation nucleus has not yet signaled for the release of acetylcholine ($ACh$), leaving the body paralyzed.
  • Hallucinations: Because the brain is still in a REM state, individuals often experience hallucinations, such as the "Old Hag" myth or alien abductions.
  • Athletes: Research suggests sleep paralysis is more common in athletes during tournaments, possibly due to a temporary depletion of acetylcholine from overexertion.
• REM Sleep Behavior Disorder: A failure of muscle atonia during REM. Individuals act out their dreams, which are often scary or violent, potentially causing injury to themselves or partners.
• Night Terrors vs. Nightmares:
  • Night Terrors: Occur during Stage 4 (deep sleep). Symptoms include screaming, sweating, and a heart rate equivalent to a $100\,m$ Olympic sprinter. The individual has no memory of a dream.
  • Nightmares: Occur during REM sleep. The individual is frightened but remembers the specific content of the dream (e.g., being chased or cheated on).