Sleep rhythms, measurement and sleep disorders
Compare circadian and ultradian rhythms, identify sleep measures, and connect internal and external timing cues to waking and sleep disorders.
Original teaching mapped to Cambridge IGCSE Psychology 0266 Version 3 for 2027–2029. The course is independent study support, not Cambridge material, an official mark scheme or an awarded grade. Named-study notes summarise the required findings; candidates are not expected to evaluate named studies. Self-check practice is not official grading, a timed Paper 1/2 simulator or teacher review.
Before you begin
- Use the Version 3 syllabus and confirm availability in your administrative zone with Cambridge and your school. Both papers are compulsory. The updated named-study summaries are required context; full original research articles are not required.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before sleep rhythms, measurement and sleep disorders, which approach is most useful?
What you will learn
- Distinguish circadian and ultradian rhythms and REM and nREM sleep
- Describe EEG, EMG, EOG and dream reports
- Explain zeitgebers, endogenous pacemakers and melatonin
- Apply rhythm concepts to jet lag, shiftwork and named disorders
Sleep cycles repeat on different time scales
A circadian rhythm repeats about every 24 hours; the sleep–wake cycle is one example. An ultradian rhythm repeats more than once in 24 hours, such as repeated changes between REM and nREM sleep. The syllabus requires the broad REM/nREM comparison, not named stages within nREM.
Sleep can be measured in different ways
EEG records electrical activity at the scalp, EMG records muscle activity and EOG records eye movement. Dream reports ask a person to describe remembered dream content. Each measure gives a different kind of evidence; a report does not directly record a dream as it occurs.
External cues and internal pacemakers coordinate timing
Zeitgebers are external cues, including light and social routines, that can shift biological rhythms. Endogenous pacemakers are internal timing mechanisms. The suprachiasmatic nucleus (SCN) helps regulate the sleep–wake rhythm; the pineal gland releases melatonin, a hormone that signals conditions favourable to sleep.
Disruption can affect sleep and everyday functioning
Jet lag follows rapid travel across time zones; shiftwork places work outside conventional daytime hours. Both can put sleep timing out of step with local cues and contribute to tiredness. Insomnia involves difficulty sleeping; sleep paralysis involves temporary inability to move when falling asleep or waking; sleepwalking involves walking while asleep. Biological rhythms may help explain timing, but symptoms alone do not establish a diagnosis.
Pause and explain
Which response best shows understanding of sleep rhythms, measurement and sleep disorders?
Worked example
After travelling across several time zones, a learner feels sleepy in the afternoon and alert late at night. Apply the rhythm account.
Show the worked solution
- The internal sleep–wake timing has not yet aligned with local time.
- Light and social schedules act as external cues.
- The mismatch can produce tiredness until timing adjusts.
Answer Jet lag can occur because the circadian sleep–wake rhythm is temporarily out of step with local time cues.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A researcher wants a measure of eye movement during sleep. Choose the appropriate tool and say what it records.
Give me a hint
Match the abbreviation to the body signal.
Compare my reasoning
- The target is eye movement.
- EOG stands for electrooculography.
- It records eye movement patterns.
Use EOG to record eye movements.
Look for these in your work
- Chooses EOG
- Identifies its signal
- Separates it from EEG/EMG
Classify a biological rhythm that repeats several times during one night.
Give me a hint
Compare the repeat interval with 24 hours.
Compare my reasoning
- The cycle repeats more than once in a 24-hour period.
- That is shorter than a daily circadian cycle.
- It is an ultradian rhythm.
Ultradian.
Look for these in your work
- Uses repetition frequency
- Compares with 24-hour cycle
- Correct term
A nurse changes from daytime shifts to overnight shifts and struggles to sleep at the intended time. Explain one mechanism and one relevant cue.
Give me a hint
Connect schedule timing to the sleep–wake rhythm.
Compare my reasoning
- The work schedule shifts activity into a different part of the day.
- The circadian rhythm may remain aligned to the old schedule.
- Light or social cues can help shift timing but adjustment may take time.
Shiftwork can desynchronise the sleep–wake cycle; light and social routines are zeitgebers that may help reset it.
Look for these in your work
- Applies circadian rhythm
- Names a zeitgeber
- Explains the mismatch
Common mistakes
- Calling REM and nREM stages of sleep that must be memorised individually.
- Confusing an EEG with eye-movement recording.
- Treating a brief symptom description as a clinical diagnosis.
What can you explain now?
Which instrument records eye movements during sleep?
Compare with the explanation
EOG
EOG records eye movements; EMG records muscle activity and EEG brain electrical activity.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do sleep rhythms, measurement tools, zeitgebers and internal pacemakers help explain sleep and waking?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.