Table 1.
Sleep-related terms used in this review
| Term | Definition |
|---|---|
| Sleep structure | The electrophysiological composition and organisation of sleep typically described in terms of the duration of defined sleep stages, the duration of the awake stage, the aggregated time spent in all sleep stages (i.e. TST), or latencies to sleep onset (see below) and other (e.g. rapid eye movement sleep onset latency) stages. When combined with measures of TIB, electrophysiogical measures can provide a reliable indication of SE (see below). Such electrophysiological measures require polysomnography (PSG), and are rarely used for 24-h or serial measurements |
| Sleep patterns | As used in this review, ‘sleep patterns’ refer to sleep–wake distributions typically assessed over units of 24 h using wrist actigraphy (and often repeated for multiple days). When combined with measures of TIB, actigraphy can provide a reliable indication of SL and SE (see below) |
| Sleep quality | An individual’s subjective experience of sleep typically focussing on problems initiating or maintaining sleep, or early morning awakening. Assessed through single items or formal psychometric evaluations, these experiences represent cardinal symptoms of insomnia. Combined with information on symptom frequency/duration and daytime symptoms (e.g. fatigue), these experiences contribute to diagnostic judgements of insomnia disorder as defined in Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-V) [49] |
| Sleep profile | Broadly, SOL, TST, and SE reported |
| Time in bed (TIB) | The time elapsed between first getting into bed (with the intention of sleeping), to the final arising |
| Sleep period time (SPT) | The time elapsed between the first onset of sleep and the final awakening |
| Wake after sleep onset (WASO) | The amount of wakefulness accumulated between the first onset of sleep and the final awakening |
| Total sleep time (TST) | The total amount of time spent asleep whilst in bed (i.e. SPT-WASO) |
| Sleep efficiency (SE) | TST expressed as a percentage of TIB: TST/TIB × 100. Whether derived from instrumental measures or subjective estimates (of TST), SE provides a sensitive metric for estimating sleep quality. A SE below 85% is indicative of disorder |
| Sleep onset latency (SOL) or sleep latency (SL) | The time elapsed between getting into bed or ‘lights out’ to sleep onset |
| Fragmentation index | A measure of the extent to which continuous sleep is interrupted by episodes of wakefulness. Sleep fragmentation is reflected in the duration and/or frequency of episodes of WASO |