Sleep Medicine and Disorders – Sleep Physiology, Common Sleep Conditions

Definition and Core Concept

This article defines Sleep Medicine as the clinical subspecialty concerned with the diagnosis and management of sleep-related conditions, including difficulties initiating or maintaining sleep, excessive daytime sleepiness, abnormal movements or behaviours during sleep, and circadian rhythm disturbances. Normal sleep is a reversible behavioural state of perceptual disengagement and unresponsiveness to environmental stimuli, characterised by distinct physiological patterns. Core features: (1) sleep architecture and stages (non-rapid eye movement – NREM stages N1, N2, N3; rapid eye movement – REM sleep), (2) sleep regulation (homeostatic drive increases with time awake, circadian rhythm regulated by suprachiasmatic nucleus and melatonin), (3) common sleep conditions (insomnia, sleep-disordered breathing, central disorders of hypersomnolence, circadian rhythm disorders, parasomnias, sleep-related movement disorders), (4) diagnostic methods (polysomnography – PSG, multiple sleep latency test – MSLT, actigraphy, sleep diaries, validated questionnaires), (5) treatment modalities (cognitive-behavioural therapy for insomnia – CBT‑I, positive airway pressure – PAP, medications, light therapy, behavioural interventions). The article addresses: stated objectives of sleep medicine; key concepts including sleep efficiency, apnoea-hypopnoea index (AHI), and insomnia severity; core mechanisms such as sleep-wake homeostasis, circadian entrainment, and upper airway collapsibility; international comparisons and debated issues (insomnia medication risks, home sleep testing accuracy, access to sleep specialists); summary and emerging trends (digital sleep therapeutics, wearable sleep trackers, orexin receptor antagonists); and a Q&A section.

1. Specific Aims of This Article

This article describes sleep medicine without endorsing specific treatments or devices. Objectives commonly cited: reducing the health consequences of untreated sleep conditions (cardiovascular, metabolic, neurocognitive, and mood effects), improving daytime function and quality of life, preventing accidents related to sleepiness, and educating the public about healthy sleep practices. The article notes that approximately 30-40% of adults report insufficient sleep or sleep difficulties, yet many sleep conditions remain underdiagnosed.

2. Foundational Conceptual Explanations

Key terminology:

• Polysomnography (PSG): Overnight sleep study recording electroencephalogram (EEG), electrooculogram (EOG), electromyogram (EMG), electrocardiogram (ECG), airflow, respiratory effort, oxygen saturation, body position, and limb movements. Gold standard for diagnosing sleep-disordered breathing and other conditions.
• Apnoea-hypopnoea index (AHI): Number of apnoeas (complete cessation of airflow lasting ≥10 seconds) plus hypopnoeas (partial reduction with oxygen desaturation or arousal) per hour of sleep. Severity categories: mild 5-15, moderate 15-30, severe >30 events/hour.
• Sleep efficiency: Total sleep time divided by time spent in bed (percentage). Normal >85%. Lower efficiency indicates fragmented or insufficient sleep.
• Insomnia: Difficulty initiating or maintaining sleep, early morning awakening, or non-restorative sleep occurring despite adequate opportunity, associated with daytime impairment (fatigue, mood changes, concentration difficulties). Duration criteria: acute (<3 months), chronic (≥3 months, ≥3 nights/week).
• Circadian rhythm: Endogenous, self-sustaining approximately 24-hour cycle regulating sleep-wake timing, hormone secretion, core body temperature, and other physiological processes. Entrained (synchronised) by environmental cues (primarily light).

Normal sleep architecture and changes with age:

• Newborns: 14-17 hours/day, 50% REM.
• Adults (18-64): 7-9 hours recommended; REM 20-25%, N3 (deep sleep) 15-20%.
• Older adults (65+): less total sleep, more frequent awakening, reduced N3 (deep) sleep, earlier circadian phase (earlier bedtime and waking).

3. Core Mechanisms and In-Depth Elaboration

Common sleep conditions (selected):

• Chronic insomnia: Most prevalent sleep condition (10-20% of adults). Risk factors: female sexs, older age, medical/psychiatric conditions, shift work, stressful life events. Persistent hyperactivity (cognitive, emotional, physiological arousal) perpetuates insomnia beyond initial triggers.
• Obstructive sleep apnoea (OSA): Repeated upper airway collapse during sleep causing oxygen desaturation, arousal, and sleep fragmentation. Risk factors: carrying excess weight (especially neck circumference), male sexs, older age, family history. Prevalence mild-moderate OSA 15-30% of adults (many undiagnosed). Associated with hypertension, cardiovascular conditions, metabolic syndrome, daytime sleepiness, and cognitive impairment.
• Central disorders of hypersomnolence (narcolepsy type 1 and type 2, idiopathic hypersomnia): Narcolepsy type 1: loss of hypocretin (orexin) neurons in hypothalamus; cataplexy (sudden muscle weakness triggered by strong emotions) characteristic. Symptoms: excessive daytime sleepiness, sleep paralysis, hypnagogic/hypnopompic hallucinations, disrupted nocturnal sleep.
• Restless legs syndrome (RLS) / Willis-Ekbom condition: Urge to move legs, usually accompanied by uncomfortable sensations, worsening at rest and in evening/night, relieved by movement. Associated with iron deficiency (low ferritin), genetic factors, renal impairment, pregnancy.
• Circadian rhythm sleep-wake disorders (delayed sleep-wake phase disorder – common in adolescents; advanced sleep-wake phase disorder – older adults; shift work disorder; non-24-hour in blind individuals).

Diagnostic pathways:

• Clinical interview: Sleep history (timing, latency, awakenings, snoring, witnessed breathing pauses, leg movements, daytime function), medical/psychiatric history, medication review, substance use (but avoid certain terms – focus on caffeine, alcohol, certain medications), sleep diary (2 weeks).
• Questionnaires: Insomnia severity index (ISI), Epworth sleepiness scale (ESS), Berlin questionnaire (OSA risk), STOP-Bang (OSA screening).
• Home sleep apnoea testing (HSAT): Portable monitor (airflow, respiratory effort, oximetry, heart rate) for moderate-to-high pretest probability of moderate-severe OSA without significant comorbidities. Less sensitive than PSG.
• Polysomnography (in-lab): Indicated for diagnostic uncertainty, suspected central sleep apnoea, hypoventilation, periodic limb movement disorder, failure of HSAT, or need for multiple studies (e.g., CPAP titration).
• Multiple sleep latency test (MSLT): Daytime nap study following PSG; measures sleep latency (time to fall asleep) and sleep-onset REM periods (SOREMPs). Used for narcolepsy and idiopathic hypersomnia.

Treatment modalities:

• Insomnia: Cognitive-behavioural therapy for insomnia (CBT‑I) – stimulus control, sleep restriction, cognitive restructuring, relaxation, sleep hygiene. Recommended first-line (effect size d=0.6-0.8, sustained >12 months). Medications: melatonin receptor agonists (ramelteon), benzodiazepine receptor agonists (eszopiclone, zolpidem), orexin receptor antagonists (suvorexant, daridorexant), sedating antidepressants (trazodone, doxepin).
• Obstructive sleep apnoea: Positive airway pressure (PAP) – continuous PAP (CPAP) is first-line, auto-adjusting PAP (APAP), bi-level PAP. Adherence (≥4 hours/night, ≥70% of nights) achieved in 40-60% of users. Alternatives: oral appliances (mandibular advancement devices) for mild-moderate OSA; weight reduction (5-10% reduction can reduce AHI by 20-30%); positional therapy; upper airway surgery (selected cases).
• Narcolepsy: Scheduled naps, behavioural modifications; wake-promoting agents (modafinil, armodafinil, solriamfetol, pitolisant); sodium oxybate (for cataplexy).
• Restless legs syndrome: Treat underlying iron deficiency (ferritin <75 μg/L – iron supplementation); alpha-2-delta ligands (gabapentin, pregabalin) first-line; dopamine agonists (pramipexole, ropinirole) but risk of augmentation (worsening symptoms).

Effectiveness evidence:

• CBT‑I meta-analysis (van Straten et al., 2018): Significant improvement in insomnia severity (standardised mean difference -0.7), sleep efficiency (+10-15%), and total sleep time (+20-30 minutes). Effects sustained at 6-12 months.
• CPAP outcomes (systematic review, Patil et al., 2019): Reduces AHI to <5 events/hour, improves Epworth sleepiness scale (by 4-6 points), reduces systolic blood pressure (by 3-5 mmHg). Higher adherence associated with larger effects.
• Orexin receptor antagonists (suvorexant, daridorexant) for insomnia: Small to moderate effects on sleep onset (reduce latency by 10-20 minutes) and wake after sleep onset (reduce by 15-25 minutes) in randomised trials. No tolerability or cognitive impairment (compared to benzodiazepine receptor agonists).

4. Comprehensive Overview and Objective Discussion

International sleep medicine resources:

Debated issues:

1. Home sleep testing vs in-lab polysomnography: HSAT less costly and more accessible but underestimates AHI (by 10-30% compared to PSG), cannot diagnose central sleep apnoea or other conditions, lacks EEG for arousal detection. For uncomplicated moderate-severe OSA, HSAT is acceptable; for complex cases, in-lab PSG remains standard.
2. Insomnia medication risks (benzodiazepine receptor agonists, sedating antidepressants): Risks of next-day sedation, falls (especially older adults), tolerance, withdrawal insomnia upon discontinuation. Guidelines recommend short-term (2-4 weeks) use or intermittent use. Many patients use chronically despite limited evidence.
3. Wearable sleep trackers (consumer devices): Estimate sleep duration and stages using accelerometry and heart rate variability. Accuracy: moderate for total sleep time (within 30-60 minutes of PSG), poor for sleep stages, variable for detecting awakening. Not medical devices; not substitutes for diagnostic testing.
4. Sleep health disparities: Prevalence of OSA and insomnia higher in certain minority populations (e.g., African American, Hispanic/Latino), with lower CPAP adherence and less access to specialty care. Contributing factors: neighbourhood noise, shift work, care access, cultural factors.

5. Summary and Future Trajectories

Summary: Sleep medicine diagnoses and manages conditions including insomnia, obstructive sleep apnoea, narcolepsy, restless legs syndrome, and circadian disorders. Polysomnography is gold standard for sleep-disordered breathing and other conditions. CBT‑I is first-line for chronic insomnia. CPAP is first-line for moderate-severe OSA. Home sleep testing is expanding access.

Emerging trends:

• Digital CBT‑I (dCBT‑I, app-based programmes): Comparable efficacy (d difference <0.1) to face-to-face CBT‑I in meta-analyses, with lower dropout. Improves accessibility.
• Orexin receptor antagonists (DORA) for insomnia: Dual orexin receptor antagonists (daridorexant, suvorexant, lemborexant) approved in US, EU, Japan. No evidence of tolerance or withdrawal; less next-day sedation than benzodiazepine receptor agonists.
• Closed-loop (adaptive) positive airway pressure devices: Adjust pressure in real-time based on flow shape, snoring, and apnoea/hypopnoea detection; improved comfort and adherence (by 10-15%).
• Hypoglossal nerve stimulation (upper airway stimulation – Inspire) for moderate-severe OSA who cannot tolerate CPAP: Implanted device stimulates genioglossus muscle to protrude tongue. Requires careful patient selection (body weight, AHI, no complete concentric collapse on sleep endoscopy). Outcomes: 60-80% responder rate (>50% reduction in AHI and <20 events/hour, n=2000+).

6. Question-and-Answer Session

Q1: How much sleep do adults need?
A: National Sleep Foundation and AASM recommend 7-9 hours per night for adults (18-64 years). Older adults (65+) also need 7-8 hours though they often sleep less due to age-related changes. Individual needs vary; adequacy assessed by feeling rested and alert, not daytime sleepiness.

Q2: What is the difference between insomnia and sleep deprivation?
A: Insomnia occurs despite adequate opportunity to sleep (time in bed); difficulty initiating or maintaining sleep or poor quality. Sleep deprivation is insufficient sleep duration due to behavioural or environmental constraints (short time in bed). Both cause daytime impairment; distinction guides treatment.

Q3: Can sleep apnoea be cured without CPAP?
A: For mild OSA, weight reduction (5-10% body weight), positional therapy (avoiding supine sleep), and oral appliances may reduce AHI to normal or mild range. For moderate-severe OSA, CPAP is most effective. Surgery (uvulopalatopharyngoplasty, maxillomandibular advancement) has variable success (20-50% cure) and is reserved for CPAP-intolerant selected patients.

Q4: What is the role of melatonin supplements?
A: Melatonin (exogenous) is effective for circadian rhythm disorders (delayed sleep phase, jet lag, shift work), reducing sleep onset latency by 15-30 minutes. For chronic insomnia without circadian delay, melatonin shows minimal benefit (sleep onset reduction <10 minutes). Doses 0.5-5 mg; higher doses not more effective. Regulated as dietary supplement, not medication, in many countries (quality variability).

https://aasm.org/ (American Academy of Sleep Medicine)
https://sleepfoundation.org/ (National Sleep Foundation)
https://www.esrs.eu/ (European Sleep Research Society)
https://www.worldsleepsociety.org/