The sleep-wake cycle: NREM and REM sleep (WACE Year 12 Psychology Unit 4)
“Sleep-wake cycle: the four stages of non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep, their characteristics (sleep state, heart rate, eye movement, muscle tension), and the length and repetition of the sleep cycle”
A night's sleep is made of repeating cycles of about 90 minutes, four to six per night. Each cycle moves through four stages of non-rapid eye movement (NREM) sleep, from light (stage 1) to very deep (stage 4), then back up into rapid eye movement (REM) sleep. In NREM sleep heart rate slows, eye movements stop and muscles relax as sleep deepens. In REM sleep the brain is highly active, heart rate becomes faster and irregular, the eyes dart rapidly and the body's muscles are almost paralysed. Deep NREM sleep dominates early in the night; REM periods lengthen towards morning.
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What this dot point is asking
The syllabus names four characteristics to describe for each stage: sleep state, heart rate, eye movement and muscle tension. You also need to describe the length and repetition of the cycle and interpret a hypnogram (a graph of sleep stages across the night).
The stages
| Stage | Sleep state | Heart rate | Eye movement | Muscle tension |
|---|---|---|---|---|
| NREM stage 1 | Very light sleep, the transition from wakefulness; easily woken; sudden muscle jerks (hypnic jerks) may occur | Begins to slow | Slow, rolling eye movements | Begins to relax |
| NREM stage 2 | Light sleep; less aware of surroundings; brief bursts of brain activity (sleep spindles and K-complexes) | Slows further | Little or none | Relaxes further |
| NREM stage 3 | Deep sleep begins; slow delta waves appear; hard to wake | Slow and regular | None | Relaxed |
| NREM stage 4 | Deepest sleep; mostly delta waves; very hard to wake; sleepwalking and sleep talking are most likely here | Slowest and most regular | None | Very relaxed, but muscles can still move |
| REM | Brain activity fast, similar to being awake; vivid dreaming; sometimes called paradoxical sleep | Faster and irregular | Rapid, darting eye movements | Very low: major skeletal muscles almost paralysed |
In REM sleep the brain is highly active and the heart and breathing rates rise, but the body is almost completely still because muscle tension drops to near zero.
Length and repetition of the sleep cycle
- One cycle, through the NREM stages and REM, takes about 90 minutes.
- Adults usually complete four to six cycles a night.
- A typical cycle: stage 1, stage 2, stage 3, stage 4, then back through stage 3 and stage 2 into REM.
- Early in the night: long periods of deep NREM sleep (stages 3 and 4) and short REM periods.
- Later in the night: deep sleep shortens or disappears, and REM periods become longer, so most REM sleep occurs in the second half of the night.
- Brief awakenings between cycles are normal and usually not remembered.
The SCSA syllabus uses the older four-stage description of NREM sleep, so use four stages in the WACE exam. Many newer sources follow a scoring system that combines stages 3 and 4 into one deep-sleep stage (N3); the deep sleep described is the same.
A student's night
A hypnogram shows a student falling asleep at 10.30 pm. Stage 4 is reached twice, in the first two cycles, at about 11 pm and about 12.30 am. After 3 am the student cycles only between stages 2 and REM, and the REM periods grow from about 10 minutes to about 30 minutes. The student wakes briefly at 5 am.
- Deep sleep early: stage 4 appears only in the first two cycles, and stages 3 and 4 are absent after 3 am.
- REM increasing: REM periods lengthen towards morning.
- Cycle length: peaks of REM recur roughly every 90 minutes.
- Brief awakening: normal between cycles.
Marker's note: describe the trend with times from the graph, then link it to the typical pattern.
- Saying the body is active during REM because the brain is
- Muscle tension is at its lowest in REM.
- Saying heart rate is lowest in REM
- It is lowest in stage 4 NREM; in REM it becomes faster and irregular.
- Saying dreaming happens only in REM
- Vivid dreaming is typical of REM, but some dreaming occurs in NREM.
- Getting the cycle timing wrong
- About 90 minutes per cycle, four to six cycles a night.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original8 marksCompare stage 4 NREM sleep and REM sleep in terms of sleep state, heart rate, eye movement and muscle tension.Show worked answer →
Two marks for each characteristic, compared across both stages.
- Sleep state
- Stage 4 NREM is the deepest sleep; the sleeper is very hard to wake and brain activity is slow. In REM sleep brain activity is fast, similar to being awake, and vivid dreaming occurs.
- Heart rate
- In stage 4 heart rate (and breathing rate) is at its slowest and most regular. In REM sleep heart rate becomes faster and irregular.
- Eye movement
- In stage 4 there is little or no eye movement. REM sleep is defined by rapid, darting eye movements under the eyelids.
- Muscle tension
- In stage 4 muscles are very relaxed but can still move (sleepwalking can occur). In REM sleep the major skeletal muscles are almost paralysed (very low muscle tension), which stops us acting out dreams.
Original4 marksDescribe how the sleep cycle changes from the first half of the night to the second half.Show worked answer →
Four marks for an accurate description.
Each cycle lasts about 90 minutes and repeats around four to six times a night. In the first half of the night, cycles contain long periods of deep NREM sleep (stages 3 and 4) and short REM periods. As the night goes on, deep sleep shortens and may disappear, while REM periods become longer, so most REM sleep occurs in the second half of the night and the sleeper spends more time in lighter stages.
Practise this
Sources & how we know this
- Psychology ATAR course Year 12 syllabus (effective from 1 January 2024) — School Curriculum and Standards Authority (SCSA)
- Psychology ATAR course: syllabus and support materials — SCSA
- Past ATAR course examinations — SCSA