Which of the following is the correct definition of sleep latency?
The proportion of time in bed actually asleep19%The period of time from the initiation of sleep to the arrival of REM19%The period of time from turning out the lights to the arrival of stage I20%The number of sleep stage changes from deeper to lighter stage or awake per hour21%The proportion of time spent in REM whilst asleep21%
It takes the average person 15-20 minutes to fall asleep, this is called sleep latency (on an EEG this is characterised by the onset of stage I sleep).
Sleep
Sleep stages
Sleep is divided into two distinct states called rapid eye movement (REM) and non-rapid eye movement (NREM). NREM is subdivided into four stages.
| Sleep stage | % of time spent in stage | EEG findings |
|---|---|---|
| I | 5% | Theta waves |
| II | 45% | K complexes and sleep spindles |
| III | 12% | <50% Delta waves |
| IV | 13% | >50% Delta waves |
It takes the average person 15-20 minutes to fall asleep, this is called sleep latency (characterised by the onset of stage I sleep). Once asleep one descends through stages I-II and then III-IV (deep stages). After about 90 minutes of sleep one enters REM. The rest of the sleep comprises of cycles through the stages. As the sleep progresses the periods of REM become greater and the periods of NREM become less. During an average night's sleep one spends 25% of the sleep in REM and 75% in NREM.
REM sleep has certain characteristics that separate it from NREM
| Characteristics of REM sleep |
|---|
| - Autonomic instability (variability in heart rate, respiratory rate, and BP) - Loss of muscle tone - Dreaming - Rapid eye movements - Penile erection - Deafness |
