Sleep latency

Sleep latency can be recorded or reported. In diagnostic sleep studies, sleep onset is defined as the first epoch of sleep on polysomnography (PSG), an overnight sleep recording [1]. Studies of healthy people have also measured it with actigraphy [2].

Self-reports and recordings can disagree. One study compared self-reported sleep latency with PSG in 33 women in the third trimester of pregnancy, 16 in the first trimester and 15 who were not pregnant [1]. All groups overestimated their sleep latency, and their estimates were closest to the first 10 minutes of uninterrupted sleep or the first slow-wave sleep, not to the first epoch of sleep [1]. The authors concluded that self-reports made by both pregnant and non-pregnant women tend to be unreliable [1].

Questionnaires also capture it. The Pittsburgh Sleep Quality Index (PSQI), a self-rated questionnaire covering a 1-month period, includes sleep latency as one of its seven component scores [3].

Normal values change with age. A meta-analysis of 65 studies covering 3,577 healthy people aged 5 to 102 found that sleep latency increased with age in adults [2]. How carefully the studies screened participants greatly changed the size of these age effects [2].

When a trial reports a shorter sleep latency, check how it was measured: PSG, actigraphy, or a questionnaire or other self-report. Self-reports and PSG recordings of the same night can disagree [1], so results from different methods are not directly comparable.

On NootropicsWork, sleep latency is graded separately from total sleep time and sleep quality. A few minutes' difference from placebo needs to be read alongside the measurement method, the population and how long participants usually took to fall asleep before the trial.

Polysomnography, actigraphy and circadian rhythm.