The Physiology of Sleep
Sleep is essential for optimal cognitive function, physical health, and emotional well-being. A typical healthy adult requires approximately 7–8 hours of sleep per night. Sleep can be divided into two main phases: non-REM (Rapid Eye Movement) and REM sleep. These phases alternate in cycles lasting about 90–120 minutes, resulting in three to four cycles per night.
Non-REM Sleep
Non-REM sleep constitutes approximately 75–80% of the total sleep time and is further subdivided into four stages:
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- Stage I: This stage marks the transition from wakefulness to sleep and accounts for 2–5% of total sleep time. During this stage, there is a decrease in alpha wave activity and an increase in theta waves on the EEG. Muscle tone also decreases, and the individual can be easily awakened.
- Stage II: Representing 45–55% of total sleep time, this stage is characterized by the appearance of sleep spindles and ‘K’ complexes on the EEG, with further reduction in muscle tone.
- Stage III: Also known as deep sleep, this stage accounts for 3–8% of sleep. It is characterized by the presence of delta waves on the EEG.
- Stage IV: The deepest and most restful stage of sleep, making up 10–15% of sleep time, is also dominated by delta wave activity.
REM Sleep
REM sleep, accounting for 20–25% of total sleep time, is characterized by rapid eye movements, increased autonomic activity, and irregularities in heart and respiratory rates. Dreaming occurs predominantly during this phase, though muscular activity is significantly decreased to prevent the physical enactment of dreams.
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Table: Differences between Non REM and REM Sleep |
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| Features | Non-REM Sleep | REM Sleep |
|---|---|---|
| Duration | 75–80% of sleep | 20–25% of sleep |
| Eye movements | No eye rolling | Rapid conjugate eye movements |
| Autonomic activity | Less autonomic activity gives slow heart rate, low BP, slow and steady respiration | Increased autonomic activity with fluctuations in BP, heart rate and respiration |
| Brain activity | Minimal | Brain is active (REM sleep is also called activated brain in a paralyzed person) |
| Muscular activity | Functional but less | Decreased. Since muscles are relaxed, snoring and OSA occurs in this stage |
| EEG | Passes from alpha to delta waves from stage I to IV | Mixed frequency, low-voltage waves with occasional bursts of saw-tooth waves |
| Dreaming | No | Yes |
Classification of Sleep Disorders
| Table : Classification of Sleep Disorders |
|
1. Insomnia
Insomnia is characterized by difficulty in initiating or maintaining sleep, often leading to daytime fatigue, impaired concentration, and mood disturbances. Insomnia can be acute or chronic.
2. Sleep-Related Breathing Disorders
These disorders involve abnormal breathing patterns during sleep and include:
- Obstructive Sleep Apnea (OSA): Characterized by the intermittent collapse of the pharyngeal airway resulting in cessation of airflow due to obstructive conditions in the nose, nasopharynx, oral cavity and oropharynx, base of tongue or larynx, leading to snoring, pauses in breathing, and daytime sleepiness. OSA is associated with increased risks of hypertension, insulin resistance, and cardiovascular disease.
- Central Sleep Apnea (CSA): In CSA, there are repeated episodes of apnea during sleep despite an open airway, the brain fails to signal the muscles to breathe. This condition is often linked to heart failure and neurological disorders.
- Mixed. It is a combination of OSA and CSA.
- Obesity Hypoventilation Syndrome (OHS): OHS involves a combination of obesity, sleep-related hypoxia, and daytime hypercapnia due to inadequate ventilation.
3. Central Disorders of Hypersomnolence
The primary condition in this category is Narcolepsy, characterized by excessive daytime sleepiness and often associated with cataplexy, which is a sudden loss of muscle tone triggered by strong emotions. Narcolepsy is linked to a deficiency of the neuropeptide hypocretin (orexin) and is diagnosed using polysomnography, the Multiple Sleep Latency Test (MSLT), and cerebrospinal fluid analysis when needed.
4. Circadian Rhythm Sleep-Wake Disorders
These disorders involve a misalignment between an individual’s sleep-wake cycle and societal norms or environmental cues. They include:
- Jet Lag
- Shift Work Disorder
- Advanced or Delayed Sleep Phase Syndromes
5. Parasomnias
Parasomnias are abnormal behaviors that occur during sleep, and they are classified into:
- Non-REM Parasomnias: These include sleepwalking, sleep talking, and sleep terrors.
- REM-Related Parasomnias: An example is REM Sleep Behavior Disorder (RBD), where individuals act out their dreams due to the lack of normal muscle atonia during REM sleep.
6. Sleep-Related Movement Disorders
These disorders include:
- Restless Legs Syndrome (RLS): Characterized by an urge to move the legs, often accompanied by unpleasant sensations that worsen during rest or in the evening.
- Periodic Limb Movement Disorder (PLMD): Involves repetitive limb movements during sleep that can disrupt overall sleep quality.
Assessment of Sleep Disorders
The assessment of sleep disorders typically involves a comprehensive medical history, sleep questionnaires such as the Epworth Sleepiness Scale, and often sleep studies.
Polysomnography (PSG) is widely recognized as the “gold standard” for diagnosing many sleep disorders, particularly sleep apnea. PSG provides comprehensive information on various physiological parameters during sleep, allowing for detailed analysis of sleep architecture, breathing patterns, and limb movements. The key components of a PSG study include:
- Electroencephalography (EEG): EEG monitors brain activity to identify different sleep stages, including non-REM and REM sleep, as well as the specific stages within non-REM sleep. This helps in assessing the overall sleep architecture and detecting abnormalities in sleep patterns.
- Electrocardiography (ECG): ECG tracks heart rate and rhythm throughout the sleep cycle, providing insights into any cardiovascular abnormalities that may occur during sleep, such as arrhythmias.
- Electrooculogram (EOM): EOM records eye movements, which are particularly important for identifying REM sleep, characterized by rapid eye movements. EOM also detects slow rolling eye movements typically seen during the transition from wakefulness to sleep.
- Electromyography (EMG): EMG measures muscle activity and is recorded from specific muscles, such as the submental (under the chin) and tibialis anterior (lower leg) muscles. This helps in detecting muscle tone and movements, which are crucial for diagnosing conditions like REM Sleep Behavior Disorder (RBD) and periodic limb movement disorder (PLMD).
- Pulse Oximetry: This non-invasive measure assesses oxygen saturation (SaO2) in the blood, which is critical for identifying episodes of desaturation associated with sleep apnea. The lowest SaO2 during sleep is particularly important for assessing the severity of the condition.
- Nasal and Oral Airflow: Sensors placed near the nose and mouth monitor airflow to detect episodes of apnea (complete cessation of airflow) and hypopnea (partial reduction in airflow). These measurements are essential for diagnosing Obstructive Sleep Apnea (OSA) and other sleep-related breathing disorders.
- Sleep Position Monitoring: Sleep position is recorded to determine whether apnea or hypopnea episodes are more frequent or severe in certain positions, such as when the individual is lying supine (on the back) versus lateral recumbent (on the side). This information can guide treatment strategies, including positional therapy.
- Blood Pressure Monitoring: Blood pressure may be continuously monitored during sleep to detect any fluctuations or abnormalities, which can be associated with sleep apnea or other sleep-related conditions.
Polysomnography’s ability to simultaneously monitor these diverse parameters makes it an invaluable tool in the comprehensive assessment of sleep disorders, leading to accurate diagnosis and effective treatment planning.
Management of Sleep Disorders
Treatment strategies for sleep disorders vary depending on the specific condition:
- Insomnia: Initial management includes sleep hygiene education and CBT-I. Short-term use of medication may be considered in some cases.
- Obstructive Sleep Apnea (OSA): Continuous Positive Airway Pressure (CPAP) therapy is the primary treatment. Other options include oral appliances, positional therapy, and surgery in select cases.
- Narcolepsy: Management involves scheduled naps, stimulant medications for daytime sleepiness, and medications to control cataplexy.
- Circadian Rhythm Disorders: These may be managed with light therapy, melatonin, and careful sleep scheduling.
- Parasomnias: Treatment often focuses on ensuring a safe sleep environment and addressing any underlying causes. Medications may be used in some cases, especially for REM Sleep Behavior Disorder.
- Restless Legs Syndrome (RLS) and Periodic Limb Movement Disorder (PLMD): Treatment may include iron supplementation if there is a deficiency, and medications such as dopamine agonists or gabapentin.
——– End of the chapter ——–
Learning resources.
- Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
- Michael A. G, Danielle E, Fishman’s Pulmonary Diseases and Disorders.
- John F Murray, Murray & Nadel’s Textbook of Respiratory Medicine
- P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
Author:
Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
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