Sleep latency is the time it takes to fall asleep after lights out. Normal is 5-20 minutes; under 5 suggests sleep deprivation; over 30 suggests insomnia or stimulant load.
Mechanism — Determined by sleep pressure (adenosine accumulation), circadian timing (melatonin onset), and arousal (cortisol, sympathetic tone). Caffeine, light exposure, and late stress all push latency longer.
Use case — Track via sleep diary or wearable. Optimal: 10-20 minutes. Shorten via better daytime light, exercise (not within 3 hours of bed), magnesium glycinate, cool dark room, consistent timing.
Caveats — Don't lie in bed past 20 minutes without sleeping — get up, low-light activity, return when sleepy. Bed-as-bedroom association matters for chronic insomnia.