Sleep
Sleep
What Do You Need to Know?
- Sleep Architecture — identify EEG patterns for each stage (spindles + K-complexes = N2; delta = N3; sawtooth waves = REM)
- Neuroanatomy — wake-promoting (orexin, NE, 5-HT, histamine, ACh) vs sleep-promoting (VLPO GABA/galanin, adenosine) systems and the flip-flop switch
- Circadian Rhythm — SCN master clock, melatonin pathway, light entrainment via melanopsin retinal ganglion cells
- Narcolepsy — Type 1 (orexin deficiency + cataplexy + HLA-DQB1*06:02) vs Type 2; MSLT criteria (≤8 min + ≥2 SOREMPs)
- RBD — loss of REM atonia, dream enactment, strong predictor of alpha-synucleinopathies (PD, DLB, MSA)
- Parasomnias — NREM (first third of night, no recall) vs REM (last third, vivid recall)
- Sleep Pharmacology — BZDs, Z-drugs, orexin antagonists, sodium oxybate, melatonin agonists
- Fatal Familial Insomnia — prion disease, thalamic degeneration, progressive insomnia → death
Sleep Architecture
Sleep Stages
| Stage | EEG Pattern | Key Features | % of Total Sleep |
|---|---|---|---|
| Wake (eyes closed) | Alpha (8–13 Hz) — posterior dominant | Beta waves when alert/eyes open | — |
| N1 | Theta (4–7 Hz); vertex sharp waves | Light sleep; slow rolling eye movements; easily aroused | ~5% |
| N2 | Sleep spindles (12–14 Hz) + K-complexes | Most abundant stage; thalamocortical spindles; memory consolidation | 45–55% |
| N3 (SWS) | Delta (<4 Hz); ≥75% of epoch | Deep/restorative sleep; GH release; hardest to arouse; NREM parasomnias arise here | 15–20% |
| REM | Low-voltage, mixed frequency; sawtooth waves | Rapid eye movements; skeletal muscle atonia; vivid dreaming | 20–25% |
Sleep Cycle Organization
- Cycle duration: ~90 minutes; 4–6 cycles per night
- First half of night: N3 (slow-wave sleep) predominates
- Second half of night: REM periods lengthen → REM increases toward morning
- REM latency: ~90 min from sleep onset (shortened in narcolepsy, depression, sleep deprivation)
- Sleep spindles → generated by thalamic reticular nucleus
- K-complexes → largest single EEG waveform; cortical response to external stimuli
Board Pearl
Sleep spindles + K-complexes = N2. N2 is the most abundant stage (~50% of total sleep). Spindles originate in the thalamic reticular nucleus. K-complexes are the largest single waveform on EEG.
Neuroanatomy of Sleep-Wake Regulation
Wake-Promoting Systems
| Structure | Neurotransmitter | Key Notes |
|---|---|---|
| ARAS (brainstem reticular formation) | Multiple (glutamate) | Ascending reticular activating system → arousal via thalamic and extrathalamic pathways |
| Locus coeruleus | Norepinephrine | Active in wake; OFF during REM |
| Raphe nuclei | Serotonin (5-HT) | Active in wake; OFF during REM |
| Tuberomammillary nucleus (TMN) | Histamine | Antihistamines → sedation; OFF during sleep |
| Basal forebrain | Acetylcholine (ACh) | Active in wake AND REM |
| Lateral hypothalamus | Orexin/Hypocretin | Stabilizes wake state; loss → narcolepsy type 1 |
Sleep-Promoting Systems
| Structure / Molecule | Neurotransmitter | Function |
|---|---|---|
| VLPO (ventrolateral preoptic area) | GABA + Galanin | Inhibits all wake-promoting centers → "sleep switch" |
| Adenosine | — | Accumulates during wakefulness (homeostatic drive); caffeine = adenosine receptor antagonist |
The Flip-Flop Switch Model
- VLPO (sleep) and wake-promoting nuclei mutually inhibit each other
- Orexin from lateral hypothalamus stabilizes the switch on the wake side
- Loss of orexin → unstable switching → intrusions of sleep into wakefulness (narcolepsy)
REM Sleep Regulation
- REM-on neurons: PPT/LDT (ACh), sublaterodorsal nucleus (glutamate)
- REM-off neurons: Locus coeruleus (NE), raphe (5-HT) — silent during REM
- REM atonia: sublaterodorsal nucleus → inhibition of spinal motor neurons via glycine/GABA; loss of this mechanism → RBD
Board Pearl
Orexin/hypocretin from the lateral hypothalamus stabilizes wakefulness. Loss of orexin neurons = narcolepsy type 1. CSF orexin <110 pg/mL (typically <90) is diagnostic. Orexin receptor antagonists (suvorexant, lemborexant) treat insomnia by blocking this system.
Clinical Pearl
Locus coeruleus (NE) and raphe nuclei (5-HT) are OFF during REM — this is why antidepressants that increase NE/5-HT (SSRIs, SNRIs, TCAs) suppress REM sleep and can treat cataplexy.
Circadian Rhythm
Suprachiasmatic Nucleus (SCN)
- Location: anterior hypothalamus, directly above the optic chiasm
- Function: master circadian pacemaker (~24.2-hour intrinsic cycle)
- Light entrainment: retinohypothalamic tract → melanopsin-containing retinal ganglion cells (intrinsically photosensitive)
- Intrinsic cycle >24 hours → requires daily light input to reset
Melatonin Pathway
Darkness → SCN → Superior Cervical Ganglion → Pineal Gland → Melatonin
- Melatonin signals darkness; suppressed by light exposure
- Peaks during nighttime; promotes sleep onset
- Core body temperature nadir: ~4 AM (coincides with peak melatonin)
Circadian Rhythm Disorders
| Disorder | Features | Treatment |
|---|---|---|
| Delayed Sleep Phase | Sleep onset 2+ hours late; common in adolescents ("night owls") | Morning bright light + evening melatonin |
| Advanced Sleep Phase | Sleep onset/wake too early; common in elderly | Evening bright light + morning melatonin |
| Non-24-Hour | Free-running rhythm; most common in totally blind patients | Tasimelteon (melatonin agonist) |
| Shift Work Disorder | Circadian misalignment with work schedule | Strategic light exposure; melatonin; modafinil |
Board Pearl
Delayed sleep phase = evening melatonin + morning bright light. Advanced sleep phase = the opposite. Non-24-hour rhythm occurs in blind patients who lack light input via melanopsin retinal ganglion cells for entrainment.
Sleep Disorders
Narcolepsy
| Feature | Type 1 (with Cataplexy) | Type 2 (without Cataplexy) |
|---|---|---|
| CSF Orexin | Low (<110 pg/mL, usually <90) | Normal |
| Cataplexy | Present — emotion-triggered transient loss of muscle tone | Absent |
| HLA association | DQB1*06:02 (>90%) | Weaker association |
| Pathophysiology | Autoimmune destruction of orexin neurons | Unknown |
| MSLT | Mean sleep latency ≤8 min + ≥2 SOREMPs | |
Narcolepsy Tetrad (Classic for Type 1)
- Excessive daytime sleepiness (present in 100%)
- Cataplexy — emotion-triggered bilateral weakness; consciousness preserved; pathognomonic
- Sleep paralysis — inability to move at sleep-wake transitions
- Hypnagogic/hypnopompic hallucinations — vivid visual hallucinations at sleep onset/awakening
Narcolepsy Treatment
| Target Symptom | Medications |
|---|---|
| Excessive Daytime Sleepiness | Modafinil/armodafinil (first-line), solriamfetol, pitolisant, methylphenidate, amphetamines |
| Cataplexy | Sodium oxybate (Xyrem/Xywav), SNRIs (venlafaxine), TCAs (clomipramine), SSRIs |
| Both EDS + Cataplexy | Sodium oxybate (treats both); consolidates nighttime sleep |
Board Pearl
Cataplexy is pathognomonic for narcolepsy type 1. Triggered by strong emotions (especially laughter). Consciousness is preserved. CSF orexin <90 pg/mL is diagnostic even without MSLT. A SOREMP on nocturnal PSG can count as one of the two required SOREMPs.
REM Sleep Behavior Disorder (RBD)
- Pathophysiology: loss of normal REM atonia → dream enactment behavior
- Clinical features: violent movements during REM sleep; punching, kicking, falling out of bed; may injure self or bed partner
- PSG finding: REM sleep without atonia (elevated chin EMG during REM)
- Alpha-synucleinopathy link: >80% develop PD, DLB, or MSA within 10–15 years
- Medication-induced RBD: SSRIs, SNRIs, TCAs can cause or worsen
- Treatment: melatonin (first-line, fewer side effects), clonazepam (low-dose); bedroom safety measures
Board Pearl
New-onset RBD in an elderly patient is a red flag for developing alpha-synucleinopathy. Longitudinal follow-up for parkinsonism and cognitive decline is essential. Do NOT dismiss as simple "bad dreams."
Restless Legs Syndrome (RLS)
- Diagnostic criteria (all required): urge to move legs + worse at rest + relieved by movement + worse in evening/night
- Associations: iron deficiency (check ferritin — treat if <75 ng/mL), uremia, pregnancy, peripheral neuropathy
- Pathophysiology: dopaminergic dysfunction + low brain iron stores
- Treatment:
- Iron supplementation if ferritin <75 ng/mL
- Alpha-2-delta ligands (gabapentin enacarbil, pregabalin) — now preferred first-line
- Dopamine agonists (pramipexole, ropinirole) — effective but risk of augmentation
- Augmentation: symptoms start earlier in the day, spread to arms, increase in intensity — caused by long-term dopamine agonist use → switch to alpha-2-delta ligand
Obstructive Sleep Apnea (OSA)
| Feature | Obstructive (OSA) | Central (CSA) |
|---|---|---|
| Mechanism | Upper airway collapse despite respiratory effort | Loss of central respiratory drive |
| Respiratory effort | Present (paradoxical chest/abdomen movements) | Absent |
| Risk factors | Obesity, large neck circumference, retrognathia, macroglossia | Heart failure (Cheyne-Stokes), opioids, brainstem lesions |
| Treatment | CPAP (gold standard), weight loss, oral appliance, surgery | Treat underlying cause; ASV (contraindicated in HFrEF) |
AHI Severity Classification
- Mild: AHI 5–15 events/hour
- Moderate: AHI 15–30 events/hour
- Severe: AHI >30 events/hour
- OSA → increased risk of hypertension, atrial fibrillation, stroke, pulmonary hypertension
- CPAP reduces cardiovascular risk and improves daytime sleepiness
Parasomnias
NREM vs REM Parasomnias
| Feature | NREM Parasomnias | REM Parasomnias |
|---|---|---|
| Timing | First third of night (N3 predominates) | Last third of night (REM predominates) |
| Recall | No memory of event (amnesia) | Vivid dream recall |
| Eyes | Open, glassy stare | Closed |
| Movement | Ambulatory, semi-purposeful | Limb jerking, punching, kicking (acting out dreams) |
| Age | Children (usually outgrown by adolescence) | Older adults (>50 years for RBD) |
| Examples | Sleepwalking, sleep terrors, confusional arousals | RBD, nightmare disorder, sleep paralysis |
| Trigger | Sleep deprivation, fever, medications | Neurodegeneration (RBD), stress (nightmares) |
Clinical Pearl
Frontal lobe epilepsy can mimic NREM parasomnias with bizarre nocturnal behaviors. Key differentiators: epilepsy episodes are stereotyped, brief, and may occur multiple times per night. Video-EEG is essential when the presentation is atypical.
Sleep Pharmacology
| Drug Class | Mechanism | Examples | Clinical Notes |
|---|---|---|---|
| Benzodiazepines | GABA-A positive allosteric modulator (non-selective) | Clonazepam, temazepam | ↑ N2; ↓ N3 and REM; tolerance/dependence risk; clonazepam used for RBD |
| Z-drugs (non-BZD hypnotics) | Selective GABA-A (α1 subunit) | Zolpidem, zaleplon, eszopiclone | Sedation with less anxiolytic effect; complex sleep behaviors (sleepwalking, sleep-eating) |
| Melatonin agonists | MT1/MT2 receptor agonists | Ramelteon, tasimelteon | Ramelteon = insomnia (sleep onset); tasimelteon = Non-24-hour disorder in blind |
| Orexin antagonists (DORAs) | Dual orexin receptor blockade | Suvorexant, lemborexant | Promotes sleep without GABA mechanism; lower abuse potential; avoid in narcolepsy |
| Sodium oxybate | GABA-B agonist / GHB | Xyrem, Xywav | Narcolepsy (EDS + cataplexy); ↑ N3; consolidates sleep; strict REMS program |
| Modafinil / Armodafinil | Wake-promoting (unclear; ? dopamine reuptake inhibition) | Provigil, Nuvigil | First-line for EDS in narcolepsy; also shift work disorder, OSA residual sleepiness |
| Gabapentinoids | α2δ calcium channel ligand | Gabapentin, pregabalin | ↑ Slow-wave sleep; first-line for RLS; useful with comorbid pain/neuropathy |
| Pitolisant | H3 histamine receptor inverse agonist | Wakix | Promotes wake by increasing histamine release; approved for narcolepsy EDS |
Board Pearl
Sodium oxybate is unique in INCREASING N3 (slow-wave sleep). Both BZDs and Z-drugs suppress N3. Sodium oxybate is the only medication that effectively treats both EDS and cataplexy in narcolepsy. It is a controlled substance (Schedule III) with a strict REMS distribution program.
Sleep and Neurological Disease
| Condition | Sleep Disturbance | Board-Relevant Points |
|---|---|---|
| Parkinson Disease | RBD, EDS, insomnia, sleep fragmentation, RLS | RBD precedes motor symptoms by >10 years; RBD predicts PD/DLB/MSA (>80% conversion) |
| Epilepsy | Sleep deprivation lowers seizure threshold; NREM activates IEDs | Frontal lobe epilepsy mimics parasomnias (stereotyped, brief, multiple/night); sleep-related epilepsies (Rolandic, ESES) |
| Alzheimer Disease | Sundowning, circadian disruption, ↓ SWS, insomnia | Loss of SCN neurons → circadian fragmentation; ↓ melatonin production; cholinergic loss disrupts sleep-wake cycling |
| Fatal Familial Insomnia | Progressive total insomnia → autonomic dysfunction → death | Prion disease (PRNP D178N-129M); thalamic degeneration (mediodorsal + anterior nuclei); autosomal dominant; no treatment |
| Kleine-Levin Syndrome | Recurrent hypersomnia (sleeping 16–20 hrs/day for days to weeks) | Adolescent males; episodes with hyperphagia, hypersexuality, cognitive/behavioral changes; self-limited episodes |
| Multiple Sclerosis | Fatigue, RLS, OSA, insomnia | Hypothalamic plaques → narcolepsy-like symptoms; central fatigue disproportionate to disability |
Board Pearl
Fatal familial insomnia is a board favorite. Key triad: progressive insomnia + dysautonomia (hyperhidrosis, tachycardia, hypertension) + motor signs. Thalamic degeneration on pathology. Do not confuse with sporadic fatal insomnia (same prion protein, different mutation/codon).
Clinical Pearl
Kleine-Levin syndrome should be suspected in an adolescent male presenting with recurrent episodes of hypersomnia lasting days to weeks, accompanied by compulsive eating and behavioral disinhibition, with complete normality between episodes.
Quick Reference
EEG Pattern → Sleep Stage
| EEG Finding | Sleep Stage |
|---|---|
| Alpha waves (8–13 Hz, posterior) | Relaxed wakefulness (eyes closed) |
| Beta waves (fast, low amplitude) | Alert wakefulness |
| Theta waves + vertex sharp waves | N1 |
| Sleep spindles (12–14 Hz) + K-complexes | N2 |
| Delta waves (<4 Hz, high amplitude) | N3 (slow-wave sleep) |
| Low-voltage mixed frequency + sawtooth waves | REM |
Clinical Clue → Diagnosis
| Clinical Scenario | Diagnosis |
|---|---|
| EDS + cataplexy + low CSF orexin | Narcolepsy type 1 |
| Elderly patient + violent dream enactment + later develops parkinsonism | REM sleep behavior disorder |
| Child + first third of night + screaming + no recall + eyes open | NREM parasomnia (sleep terror) |
| Urge to move legs + worse at rest + worse at night + low ferritin | Restless legs syndrome |
| Snoring + witnessed apneas + EDS + obesity + hypertension | Obstructive sleep apnea |
| Adolescent cannot fall asleep until 2–3 AM, sleeps normally otherwise | Delayed sleep phase disorder |
| Blind patient with free-running sleep-wake rhythm | Non-24-hour sleep-wake disorder |
| Progressive insomnia + dysautonomia + thalamic degeneration | Fatal familial insomnia |
| Adolescent male + recurrent hypersomnia + hyperphagia + behavioral changes | Kleine-Levin syndrome |
| RLS symptoms worsening earlier in day on dopamine agonist | Augmentation → switch to alpha-2-delta ligand |
Neurotransmitter Quick Reference
| Neurotransmitter | Source | Role in Sleep-Wake |
|---|---|---|
| Orexin/Hypocretin | Lateral hypothalamus | Stabilizes wakefulness; loss = narcolepsy type 1 |
| GABA + Galanin | VLPO | Promotes sleep by inhibiting wake centers |
| Norepinephrine | Locus coeruleus | Wake-promoting; OFF in REM |
| Serotonin | Raphe nuclei | Wake-promoting; OFF in REM |
| Histamine | TMN | Wake-promoting; antihistamines → sedation |
| Acetylcholine | Basal forebrain, PPT/LDT | Active in wake AND REM; REM-on neurotransmitter |
| Adenosine | Diffuse (metabolic byproduct) | Homeostatic sleep drive; caffeine blocks its receptors |
References
- Kryger MH, Roth T, Dement WC. Principles and Practice of Sleep Medicine. 7th ed. Elsevier; 2022.
- American Academy of Sleep Medicine. International Classification of Sleep Disorders. 3rd ed, Text Revision (ICSD-3-TR). AASM; 2023.
- Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor's Principles of Neurology. 12th ed. McGraw-Hill; 2023.
- Continuum (Minneap Minn). Sleep Neurology. American Academy of Neurology; 2023;29(4).
- Scammell TE. Narcolepsy. N Engl J Med. 2015;373(27):2654–2662.
- Iranzo A, et al. Neurodegenerative disease status and post-mortem pathology in idiopathic REM sleep behaviour disorder. Lancet Neurol. 2013;12(5):443–453.