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| Term | Definition |
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| Sleep disturbance | Sleep pattern divergent of what is considered to be normal as objectively measured, for example, by polysomnography. |
| Sleep disorder | Medical disorder involving sleep, resulting in suffering or reduced functioning, including dyssomnias and parasomnias. |
| Sleep onset latency | Time interval between time of turning of the lights and onset of sleep. |
| Sleep efficiency | Ratio of the time spent asleep to the amount of time spent in bed. |
| Chronotype | Individual internal timing type regarding preferred time for mental and physical activity and sleep. |
| Homeostatic sleep drive | Drive to sleep that gradually increases with prolonged wakefulness and decreases during sleep. |
| Sleep fragmentation | Disrupted sleep cycle due to interruption of a sleep stage, as a result of the appearance of a lighter sleep stage or wakefulness. |
| Sleep phase advance | Forward shift of the sleep/wake rhythm, as demonstrated by the time of the nocturnal elevation of plasma melatonin. |
| Insomnia | Sleep disorder comprising difficulty initiating and/or maintaining sleep or nonrestorative sleep for at least one month, resulting in significant distress and/or impaired daytime functioning |
| REM sleep behavior disorder | Parasomnia characterized by “acting out” of dreams during REM sleep due to absence of normally occurring muscle atonia. |
| Excessive daytime sleepiness | Parasomnia characterized by excessive sleepiness during the day, often with hypersomnia and the occurrence of sleep attacks. |
| Periodic limb movement disorder | Sleep disorder characterized by involuntary limb movements causing fragmented sleep. |
| Restless legs syndrome | Syndrome characterized by unpleasant sensations in one or more limbs, exacerbated by rest and relieved with activity, paired with a strong urge to move the affected limbs, often with paresthesias or dysesthesias. |
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