Sleep and Waking

Homeostatic Systems - Sleep and Waking

Definition of Sleep

  • Sleep is defined as a naturally recurring state which is characterized by:   - Reduced or absent consciousness.   - Relatively suspended sensory activity.   - Inactivity of nearly all voluntary muscles.

  • Sleep is a phenomenon observed in:   - All mammals.   - Birds.   - Most reptiles, amphibians, and fish.

  • Humans spend approximately one-third of their lives sleeping, which averages to around 7.5 hours daily.

Possible Functions of Sleep

  • Restoration: Sleep serves various restorative functions in the body.   - Sleep deprivation has significant adverse effects on the immune system.   - Sleep may increase endogenous antioxidants, which help combat the accumulation of free radicals in the brain.

  • Energy Conservation:   - Sleep helps replenish brain glycogen.   - It contributes to reducing heat loss during colder nights, although the reduction in metabolism is only around 5-10%.

  • Memory Consolidation: Sleep plays a critical role in processing and consolidating memories.

  • Ecological Adaptation: Searching for food and avoiding predators becomes challenging when relying solely on vision in darkness.

Effects of Sleep Deprivation

  • Sleep deprivation results in various negative effects, including:   - Irritability   - Cognitive impairment   - Memory lapses or memory loss   - Impaired moral judgment   - Severe yawning   - Hallucinations   - Symptoms similar to Attention Deficit Hyperactivity Disorder (ADHD)   - Impaired immune system functionality   - Increased heart rate variability, which can lead to a higher risk of heart disease   - Increased reaction time, with decreased accuracy in tasks   - Physical symptoms such as tremors and aches   - Increased risk of developing Type 2 diabetes   - Growth suppression   - Increased risk of obesity   - Decreased body temperature

Essential Nature of Sleep

  • Sleep is essential for overall health and well-being.

  • Evidence indicates that sleep deprivation can lead to death under extreme circumstances. Experimental studies show that while control animals can sleep intermittently (sleeping when they choose), experimental animals that are deprived of sleep tend to lose body weight despite increased food intake.

Patterns of Sleep in Different Species

  • Some mammals, such as rabbits and giraffes, sleep for only short periods of time (a few minutes).

  • Other species like dolphins and seals can sleep with one hemisphere of their brain at a time instead of sleeping with both hemispheres at once.

Rhythms of Sleep

  • In chronobiology, various types of rhythms exist:   - Ultradian Rhythms: These are recurrent periods or cycles that repeat throughout a 24-hour day, such as feeding and respiration rates.   - Circadian Rhythms: These rhythms complete one cycle daily, for instance, the sleep-wake cycle and body temperature variations.   - Infradian Rhythms: These have periods longer than a day, such as the human menstrual cycle or seasonal variations.   

Circadian Rhythm in Absence of External Cues

  • Circulatory functions influenced by circadian rhythms include:   - Sleep-wake cycles.   - Body temperature fluctuations.   - Electrolyte levels in the body.   - Some hormone secretions.

  • Human's circadian rhythm may persist beyond 24 hours under certain conditions, averaging around 26+ hours without external alignment cues.

Daily Fluctuations Influenced by Circadian Rhythms

  • High alertness usually occurs around noon.

  • Various physiological changes occur throughout the day:   - 08:30: Highest testosterone secretion and increased likelihood of bowel movements.   - 07:30: Melatonin secretion comes to a stop.   - 06:45: There's a sharp rise in blood pressure.   - 04:30: Lowest body temperature typically recorded.   - 02:00: Deepest sleep period observed.   - 14:30: Best coordination is experienced.   - 15:30: Fastest reaction time detected.   - 17:00: Greatest cardiovascular efficiency and muscle strength identified.   - 19:00: Highest observed body temperature.   - 21:00: Melatonin secretion begins.

Suprachiasmatic Nucleus (SCN) - The Master Clock

  • The suprachiasmatic nucleus, located in the hypothalamus, is often referred to as the body’s master clock, consisting of about 20,000 neurons, roughly the size of a grain of rice. It regulates:   - Circadian rhythmicity of sleep.   - Physical activity levels.   - Alertness patterns.   - Hormonal levels in the body.   - Body temperature.   - Immune function and digestive activities.

  • Neurons in the ventrolateral SCN are responsive to light-induced gene expression, receiving projections from retinal ganglion cells and relaying light stimulation throughout the SCN, a process known as entrainment.

Melatonin - The "Dracula of Hormones"

  • Melatonin is a hormone released primarily at night, characterized by:   - Decreased motor activity.   - Induction of fatigue.   - Lowering of body temperature.

  • Released into the bloodstream as an endocrine hormone, its secretion is influenced significantly by light exposure to the eyes. Under dark conditions, the SCN activates the pineal gland to produce melatonin, typically starting around 9 PM and lasting for about 12 hours.

Sleep Measurement Techniques

  • Sleep is generally measured using several techniques:   - Electroencephalogram (EEG): Records electrical activity in the brain.   - Electro-oculogram (EOG): Measures eye movements during sleep.   - Electromyogram (EMG): Assesses muscle tone during different sleep stages.

Sleep Cycle Stages

  • Sleep is divided into various stages, comprising a complete cycle of sleep:   - Stage 1: Light sleep, characterized by reduced muscle activity and occasional muscle twitching (4-5% of total sleep time).   - Stage 2: Represents deeper light sleep, with slowed breathing and heart rate plus minor decreases in body temperature (45-55% of sleep time).   - Stage 3: Beginning of deep sleep where the brain starts generating slow delta waves (4-6% of sleep).   - Stage 4: Very deep sleep where rhythmic breathing and delta wave production increase (12-15% of sleep).   - Stage 5 (REM sleep): Rapid eye movement sleep where brainwaves speed up, dreaming occurs, muscles relax, heart rates increase and breathing becomes rapid and shallow (20-25% of sleep).

EEG Rhythms Associated with Sleep

  • Drowsiness is marked by theta activity (3.5-7.5 Hz).

  • Slow-wave sleep is signified by the presence of delta waves.

  • Sleep spindles observed at a frequency of 10-12 Hz indicate non-REM sleep, whereas REM sleep shows brain activity patterns similar to those of an awake individual with significant decreases in motor activity due to GABAergic cell suppression in the pedunculopontine nucleus (part of the reticular formation).

REM Sleep Characteristics

  • REM sleep (also known as paradoxical sleep) is characterized by:   - EEG patterns that resemble that of an awake state, despite reduced motor output because of inhibition of motor activity.   - It is an active brain state resulting in higher energy consumption during this phase.   - Each night, about 2 hours are spent in both REM and non-REM sleep with:     - Non-REM sleep characterized by shorter, less vivid dreams that are related to routine daily thoughts (often referred to as Freud's "day's residue").     - REM sleep involving longer, more vivid, emotional, and often bizarre dreams.

Causal Factors Modulating Sleep-Inducing Circuits

  • Several brain regions regulate sleep, including:   - Amygdala: involved in processing emotions during REM sleep.   - Parahippocampus: plays a role in memory consolidation.   - Pontine Tegmentum: significant during REM sleep.   - Anterior Cingulate: less active during REM.

  • An increase in limbic system activity occurs during REM sleep, while the prefrontal cortex which usually suppresses inappropriate responses becomes less active, possibly explaining the emotional nature of dreams.

Mechanisms of Motor Output Suppression during REM Sleep

  • REM sleep is notable for decreased motor output (REM atonia) which is controlled by GABAergic neurons in the pedunculopontine nucleus. This process involves suppression of lower motor neurons in the spinal cord, initiated via projections through the pons. Additionally, sensory response diminishes due to the suppression of the dorsal column nuclei by pedunculopontine neurons.

Circuitry for Sleep and Wakefulness

  • The reticular activating system is crucial for regulating wakefulness and sleep states:   - Stimulation of cholinergic neurons in the pons and midbrain awakens animals, triggering pontine-geniculo-occipital (PGO) waves associated with eye movements in REM sleep.   - Conversely, low-frequency stimulation of the thalamus prompts sleep in cats.

Neurotransmitter Implications in Arousal & Sleep

  • Cholinergic nuclei in the pons and midbrain show high levels of activity during wakefulness. Their low activity leads to the onset of non-REM sleep, with increased cholinergic activity accompanying REM sleep.

  • Serotonergic neurons in the raphe are active during waking states but decrease in activity during sleep stages.

  • Noradrenergic projections from the locus coeruleus are active during arousal but inactive during sleep periods.

  • The histamine neurons found in the tuberomammillary nucleus promote wakefulness, showcasing rapid firing patterns while awake, slow firing during relaxation, and cessation during sleep. This functional characteristic explains the sedative effects of antihistamines.

  • Orexin, produced in the lateral hypothalamus, modulates the activity of histamine neurons and plays a significant role in promoting wakefulness and appetite.

Orexin and Sleep Stability

  • Orexin neurons interact with various brain nuclei involved in maintaining wakefulness, including dopamine, norepinephrine, histamine, and acetylcholine systems.

  • The destruction of orexin neurons is associated with narcolepsy, a sleep disorder characterized by sudden sleep attacks.

Summary of Sleep Regulation Mechanisms

  • Sleep regulation involves both monoaminergic and cholinergic systems:   - These systems are active during the waking state.   - Their reduced activity initiates non-REM sleep.   - During REM sleep, activity levels of both monoaminergic and serotonergic neurons decrease, while cholinergic activity matches levels characteristic of the waking state.

Adenosine and Its Role in Sleep Regulation

  • The relationship between adenosine levels and sleep regulation is characterized as follows:   - Increased adenosine correlates with decreased levels of histamine and orexin, facilitating sleep.   - Conversely, decreased adenosine activity enhances the activity of dopamine and glutamate, promoting wakefulness.   - Caffeine acts as an antagonist of adenosine, effectively blocking its effects and hindering sleep.

Sleep Disorders

  • Sleep disorders can be defined practically by responses to the questions:   - “Do you experience difficulty sleeping?”   - “Do you have difficulty falling or staying asleep?”

  • Narcolepsy is characterized by:   - Sleep attacks with sudden loss of muscle control (cataplexy).   - Individuals may quickly enter REM sleep (within around 5 minutes).   - Associated mutations are typically found in orexin receptors.

Pharmacological Interventions for Sleep Disorders

  • Sleeping pills (hypnotics) increase GABA activity and overall inhibition but may possess multiple side effects:   - Habit forming tendencies.   - Disruptions in natural sleep patterns.

  • Common hypnotics include benzodiazepines (non-narcotics) and narcotics which influence GABA and sodium channel activity.