The time from lights-out to sleep onset. Clinical normal range is 10–20 minutes. Both extremes carry diagnostic significance.
Sleep latency is the time elapsed between the moment a person attempts to sleep (lights off) and the onset of sleep (first 30 seconds of stage N1 or N2 on PSG). It is measured in minutes.
Sleep latency is a core component of insomnia assessment and is used in the Multiple Sleep Latency Test (MSLT) to quantify daytime sleepiness. The MSLT considers sleep latency under 5 minutes as indicating pathological sleepiness.
| Latency | Category | Clinical interpretation |
|---|---|---|
| < 5 min | Very short | Suggests significant sleep deprivation or sleep disorder |
| 5–10 min | Short | Mild sleep pressure, possibly mildly sleep-deprived |
| 10–20 min | Normal | Clinically typical for healthy rested adults |
| 20–30 min | Extended | May indicate mild insomnia or anxiety |
| > 30 min | Prolonged | Clinical insomnia threshold (if persistent) |
Source: AASM International Classification of Sleep Disorders, 3rd ed. 2014.
Consumer wearables detect sleep onset using accelerometry (movement cessation) combined with heart rate changes. This tends to underestimate sleep latency — wearables often record sleep onset several minutes before actual PSG-confirmed onset because they detect stillness rather than brain state. Mean error is typically 5–10 minutes.