Sleep
Sleep
What You'll Learn
- 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
HighYield Pearls
- Sleep spindles + K-complexes: hallmark of N2 (most abundant stage, ~50%); spindles 12–14 Hz generated by thalamic reticular nucleus
- Sawtooth waves + atonia + rapid eye movements: REM — paradoxical EEG resembles wake; REM lengthens through the night
- Delta (0.5–2 Hz) ≥20% of epoch: N3 slow-wave sleep; dominates first third of night; declarative memory consolidation + GH release + glymphatic clearance of β-amyloid/tau
- VLPO (ventrolateral preoptic): GABA/galanin sleep switch — inhibits all wake centers; flip-flop with orexin/monoamines
- Orexin/hypocretin loss (lateral hypothalamus) → narcolepsy type 1 (CSF orexin <110 pg/mL + cataplexy + HLA-DQB1*06:02); MSLT mean ≤8 min + ≥2 SOREMPs
- REM sleep without atonia on PSG = RBD — isolated PSG-confirmed RBD is one of the strongest prodromal α-synucleinopathy markers: ~6–8% per year, ~70–75% by 12 years in large multicenter cohorts; can exceed 80% with longer follow-up/older cohorts. AASM 2023: environmental sleep safety FIRST; immediate-release melatonin and clonazepam are both conditionally recommended (melatonin often preferred in older/cognitively impaired/OSA/fall-risk patients).
- SCN (suprachiasmatic n.): master circadian clock; entrained by melanopsin retinal ganglion cells → pineal melatonin peaks 3–5 AM; adenosine drives homeostatic sleep pressure (caffeine blocks A1/A2A)
- NREM parasomnias (sleepwalking, terrors, confusional arousals) → first third of night, arise from N3, no recall, childhood/family hx; REM parasomnias (RBD, nightmares) → last third, vivid recall
- Aging: phase advance + ↓N3 + ↑N1 + fragmentation + earlier wake; newborns ~50% REM, ~16 h total
- REM suppressants: alcohol, SSRIs, TCAs; prazosin for PTSD nightmares; for chronic insomnia, CBT-I is first-line — DORAs (suvorexant, lemborexant, daridorexant) are pharmacologic options when medication is needed, NOT co-first-line with CBT-I
🔍 Quick ReferenceStage / EEG · Neuroanatomy / NT · Function / lifespan
Stage / EEG features
- Posterior dominant α rhythm 8–13 Hz, eyes closed → relaxed wake
- Vertex sharp waves + slow rolling eye movements + θ activity → N1 (~5%, lightest stage)
- Sleep spindles (12–14 Hz, central) + K-complexes → N2 (~50%, most abundant)
- High-amplitude δ (0.5–2 Hz, ≥75 µV) ≥20% of epoch → N3 slow-wave sleep
- Sawtooth waves + low-voltage mixed-frequency EEG + EMG atonia + REMs → REM sleep (paradoxical sleep)
- SOREMP (sleep-onset REM period) on MSLT, mean latency ≤8 min, ≥2 SOREMPs → narcolepsy
- ~90-min cycle, REM lengthens through night, N3 front-loaded → normal adult sleep architecture
Neuroanatomy / neurotransmitter control
- ARAS (pontomesencephalic reticular formation) → thalamus → cortex → arousal/wake
- Locus coeruleus (NE) + dorsal raphe (5-HT) + tuberomammillary n. (histamine) + basal forebrain (ACh) → monoaminergic/cholinergic wake-promoting network (NE + 5-HT silent in REM)
- Lateral hypothalamic orexin/hypocretin neurons → stabilize wake (loss → narcolepsy type 1)
- VLPO GABA/galanin neurons → sleep switch — flip-flop inhibition of all wake centers
- PPT/LDT cholinergic neurons → REM-on generators driving sublaterodorsal nucleus (SLD)
- Ventral medullary (magnocellular) glycinergic neurons → REM atonia via motor-neuron inhibition
- SCN (suprachiasmatic n., anterior hypothalamus) + retinohypothalamic tract + pineal melatonin (peak 3–5 AM) → circadian drive
- Adenosine accumulation (blocked by caffeine at A1/A2A) → homeostatic sleep pressure
Function / lifespan / disease
- Cataplexy + EDS + sleep paralysis + hypnagogic hallucinations + CSF orexin <110 pg/mL + HLA-DQB1*06:02 → narcolepsy type 1
- Dream enactment + REM sleep without atonia on PSG → REM sleep behavior disorder (RBD) — prodromal α-synucleinopathy (PD/DLB/MSA); conversion ~6–8%/yr, ~70–75% by 12 yr, can exceed 80% with longer follow-up
- Recurrent hypersomnia + hyperphagia + hypersexuality in adolescent males → Kleine-Levin syndrome
- Progressive insomnia + dysautonomia + thalamic degeneration (PRNP mutation) → fatal familial insomnia
- Sleepwalking / sleep terrors / confusional arousals, first third of night, no recall → NREM parasomnia (arises from N3)
- Declarative memory consolidation in N3; procedural/emotional in REM; glymphatic clearance of β-amyloid & tau → function of sleep
- Newborn ~16 h sleep with ~50% REM; elderly → phase advance + ↓N3 + fragmentation → lifespan changes
- Eastward jet lag worse; DSPD in adolescents (AM bright light + PM melatonin); ASPD in elderly (PM light + AM melatonin); shift-work disorder → modafinil → circadian rhythm disorders
- Alcohol / SSRIs / TCAs suppress REM; prazosin for PTSD nightmares; CBT-I is first-line for chronic insomnia (ACP/AASM); DORAs (suvorexant/lemborexant/daridorexant) are pharmacologic options when medication is needed — NOT co-first-line with CBT-I → sleep pharmacology pearls
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 (0.5–2 Hz, ≥75 µV peak-to-peak), ≥20% of epoch (AASM scoring) | 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) | Cortical-activating cholinergic source; active in wake AND REM |
| Pedunculopontine + laterodorsal tegmental nuclei (PPT/LDT) | Acetylcholine (ACh) | Brainstem cholinergic wake/REM generators (distinct from basal forebrain); drive thalamocortical activation and REM-on circuitry |
| 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 → ventromedial medulla → glycinergic (and GABAergic) inhibition of spinal alpha motor neurons; 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.
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