CourseCourseQ BankQuestion BankDocsDocuments

C3.1.11—Melatonin and sleep

Melatonin and sleep links named stimuli, coordinating structures and effectors so body systems or plant tissues produce an integrated biological response.

Syllabus
First assessment 2025
Objective
C3.1.11
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1

Common command terms

  • Explain
  • Identify
  • Describe
  • State
  • Deduce
  • Outline

Scoring notes

Common mistake
Describing melatonin as a general energy hormone instead of a sleep-timing signal.

Recent exam appearances

November 2025Paper1A ["HL"] · TZ327[ 1 ]C3.1.11—Melatonin and sleep
May 2025Paper1A ["HL"] · TZ228[ 1 ]C3.1.11—Melatonin and sleep
May 2023Paper2 ["HL"] · TZ12(d)[ 2 ]C3.1.11—Melatonin and sleep
November 2022Paper1 ["HL"] · TZ025[ 1 ]C3.1.11—Melatonin and sleep
May 2022Paper2 ["HL"] · TZ24(c)[ 1 ]C3.1.11—Melatonin and sleep
Practice this objective

Coverage 2016–2025 · Updated 16 Jul 2026

Melatonin links darkness to the body clock

Melatonin links darkness to the body clock.

Melatonin released by the pineal gland rises in darkness and helps synchronise circadian timing, including sleep propensity. Light reaching the retina can suppress its release.

light input → clock pathway → pineal melatonin → timing effect.

A bright screen late at night can delay the melatonin rise, shifting the timing signal even if the person feels tired.

Melatonin is a timing hormone, not an instant anaesthetic; sleep also depends on behaviour and other signals.

Melatonin and sleep

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Identify / Describe.

Command terms

Explain / Identify / Describe / State / Deduce / Outline

What earns marks

Mark schemes reward named structures, correct direction of information flow and explanations that keep the sequence causal.

Watch for

Describing melatonin as a general energy hormone instead of a sleep-timing signal.

Representative question

Question 1

[Maximum number: 2]

Outline the role of melatonin in humans.

Body-System Integration

  • Nervous signals, hormones and blood transport integrate organs into coordinated systems; emergent functions arise from their interactions.
  • Sensory neurons carry receptor input to the CNS; motor neurons carry output to effectors. Mixed nerves contain both fibre types.
  • Reflex arcs provide rapid involuntary responses through sensory, relay and motor neurons. The spinal cord also links the brain and peripheral nerves.
  • Cerebral hemispheres support conscious processing; the cerebellum coordinates movement and balance; the medulla adjusts ventilation and heart activity.
  • The hypothalamus links nervous and endocrine control through the pituitary. Pineal melatonin helps time sleep; adrenal epinephrine supports acute stress responses.
  • Baroreceptors and chemoreceptors provide feedback about pressure, CO2, pH and O2.
  • The CNS controls voluntary swallowing and egestion, while the enteric nervous system coordinates gut peristalsis.

Concept essentials

  • Melatonin and sleep depends on linking the stimulus to the coordinating structure or signal.
  • The pathway for melatonin and sleep must keep information flow and response direction clear.
  • Named tissues, hormones, neurons or effectors give melatonin and sleep its biological specificity.
  • Evidence about melatonin and sleep is strongest when structure, mechanism and outcome are connected.