1 Psychology and human development1.2 Awareness, sleep and consciousness

1.2.3 Sleep timing and deprivation

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Written by Resident Doctor Committee · Published

New in the 2027 syllabus

The College added this topic for exams from January 2027.

How sleep is regulated

  • Circadian regulation sets how sleepy we are across roughly 24 hours. The body clock is the suprachiasmatic nucleus, kept in step with the day mainly by light.

  • Homeostatic sleep pressure builds up while we are awake and falls during sleep. It works alongside circadian timing rather than replacing it.

  • Melatonin signals biological night. Its timing matters more than treating it as a simple on-off switch for sleep.

  • Ultradian cycles: sleep alternates between non-rapid eye movement (NREM) and rapid eye movement (REM) sleep several times a night. Deeper sleep comes earlier, and REM periods get longer later.

Schematic two-process model with an illustrative 23:00 to 07:00 sleep schedule. Sleep pressure depends on time awake, while circadian timing changes the drive for wakefulness; the curves use separate arbitrary scales and are not clinical recordings. Original illustration drawn in Python for MRCPsychiatrist; see images/ATTRIBUTION.md

Dreams and unusual sleep behaviour

  • Dreaming happens in both REM and NREM sleep, but vivid, story-like dreams are more often recalled from REM sleep.

  • In REM sleep behaviour disorder, the normal muscle paralysis of REM sleep (atonia) is lost, so the person acts out their dreams. Sleepwalking is different: it arises from NREM sleep.

Sleep loss

  • Sleep deprivation impairs sustained attention, reaction time, working memory and emotion regulation. This leads to more errors and a higher risk of accidents.

  • Microsleeps are brief, involuntary episodes of sleep. They can interrupt behaviour in someone who seems awake.

  • Chronic sleep restriction can cause impairment that keeps building, even though the person feels used to it.

Revision material for the MRCPsych examinations. Not clinical guidance, and not for use in patient care.