Sleep latency
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"sleep latency"[MeSH Terms] AND measurement
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Sleep Latency
Definition
Sleep latency is the time elapsed from "lights out" (or the decision to sleep) to the onset of sleep, as determined on polysomnography (PSG) or the Multiple Sleep Latency Test (MSLT). It is one of the key parameters scored in any PSG study, alongside:
- Total sleep period - time from sleep onset to final awakening
- Total sleep time - total time asleep (excluding waking periods)
- REM sleep latency - time from sleep onset to the first REM episode
- Sleep efficiency - total sleep time / total time in bed × 100%
- Wake after sleep onset (WASO) - time spent awake after initial sleep onset
- Bradley and Daroff's Neurology in Clinical Practice, p. 2422
Normal Values
| Parameter | Normal / Reference Value |
|---|---|
| Sleep latency (PSG) | ~10-20 minutes in healthy adults |
| Pathological sleepiness threshold | < 5 minutes |
| MSLT mean sleep latency (narcolepsy cutoff) | < 8 minutes |
| MWT mean sleep latency | 30.4 ± 11.2 minutes |
| MWT abnormal threshold | < 8 minutes (definitely abnormal) |
| Insomnia criteria (subjective) | > 30 minutes |
Measurement Tools
1. Polysomnography (PSG)
The gold standard. Sleep latency is scored per the AASM Manual for the Scoring of Sleep and Associated Events. Scoring must ideally be performed manually, as computerized scoring is unreliable.
2. Multiple Sleep Latency Test (MSLT)
- Assesses daytime sleepiness - the tendency to fall asleep.
- Protocol: 4-5 nap opportunities every 2 hours, starting 2 hours after morning awakening. Each nap session is 20 minutes maximum.
- The patient is instructed not to resist sleep.
- Mean sleep latency < 8 minutes + ≥2 sleep-onset REM periods (SOREMPs) strongly suggests narcolepsy.
- One SOREMP on the preceding overnight PSG (REM latency ≤15 min) can substitute for one MSLT SOREMP.
- - Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 6542; Bradley and Daroff, p. 2427
3. Maintenance of Wakefulness Test (MWT)
- Assesses the ability to stay awake (not the tendency to fall asleep).
- Protocol: 40-minute sessions at 2-hour intervals. Patient is instructed to resist sleep.
- Used for treatment monitoring and "fit for duty" assessments (e.g., commercial drivers, pilots).
- Sleep latency < 8 minutes = abnormal; 8-40 minutes = uncertain significance.
- Mean latency in normals: 30.4 ± 11.2 minutes; upper 95% CI = 40 minutes.
- - Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 6542
Clinical Significance
Insomnia
Sleep latency > 30 minutes is a core criterion for insomnia, alongside:
- Wakefulness after sleep onset (WASO) > 30 min
- Sleep efficiency < 85%
- Total sleep time < 6.5 hours
- Stahl's Essential Psychopharmacology, p. 1029
Narcolepsy
The DSM-5/ICSD diagnostic criteria require:
- Mean MSLT sleep latency ≤ 8 minutes, AND
- ≥2 SOREMPs (or 1 SOREMP on MSLT + REM latency ≤15 min on overnight PSG)
Low CSF hypocretin-1 (≤110 pg/mL or ≤1/3 of reference values) can confirm narcolepsy type 1 independently.
- Goldman-Cecil Medicine; Kaplan & Sadock's, p. 6506
Excessive Daytime Sleepiness (EDS)
Short sleep latency on MSLT is the objective marker of EDS from any cause, including:
- Obstructive sleep apnea (OSA)
- Insufficient sleep syndrome
- Circadian rhythm disorders
- Central hypersomnias (narcolepsy, idiopathic hypersomnia)
Key Distinction: MSLT vs MWT
| Feature | MSLT | MWT |
|---|---|---|
| Instruction | Fall asleep | Stay awake |
| Session length | 20 min | 40 min |
| Measures | Sleepiness tendency | Ability to maintain wakefulness |
| Primary use | Diagnose narcolepsy/hypersomnia | Assess treatment response; fitness for duty |
| Normal mean latency | ~15-18 min | ~30.4 min |
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