What an MSLT measures
After a documented night of sleep, the MSLT presents the patient with five 20-minute nap opportunities at two-hour intervals. We measure how quickly the patient falls asleep in each opportunity, and whether REM sleep appears within 15 minutes of sleep onset.
Diagnostic thresholds
- Mean sleep latency ≤ 8 minutes is consistent with significant excessive daytime sleepiness
- Two or more sleep-onset REM periods support a diagnosis of narcolepsy
- Less than two sleep-onset REM periods can indicate idiopathic hypersomnia
- Results are interpreted in the context of the prior overnight polysomnogram
Preparation matters
- A full polysomnogram the night before
- Two weeks of sleep diary or actigraphy to verify adequate prior sleep
- Medication adjustments — many sleep-affecting drugs must be tapered before the test, in coordination with your prescribing physician
- Avoid caffeine and stimulants on the day of testing
The day of testing
- The overnight polysomnogram concludes at 06:30
- The first nap opportunity is at approximately 09:00
- Subsequent opportunities are at 11:00, 13:00, 15:00, and 17:00
- Between naps, patients remain awake; reading and quiet activity are fine
- Each nap opportunity is 20 minutes (or 15 minutes after sleep onset)
What MSLT can and cannot do
MSLT is specifically designed for narcolepsy and idiopathic hypersomnia evaluation. It is not the right test for fatigue without sleepiness, for circadian rhythm disorders, or for behavioral sleep deprivation — each has its own diagnostic pathway.
The MSLT is a precise instrument. It rewards careful preparation; preparation is half the test.— Seoul Sleep Center diagnostic protocol