Where is the master circadian pacemaker located in the human brain?
A: Pineal gland, in the epithalamus
B: Suprachiasmatic nucleus (SCN), in the anterior hypothalamus
C: Locus coeruleus, in the pons
D: Dorsal raphe nucleus, in the midbrain
Correct: Suprachiasmatic nucleus (SCN), in the anterior hypothalamus
The suprachiasmatic nucleus (SCN) — a paired cluster of roughly 20,000 neurons in the anterior hypothalamus, just above the optic chiasm — is the master circadian pacemaker. SCN neurons contain an autonomous molecular clock (CLOCK/BMAL1–PER/CRY feedback loop) that generates a ~24-hour rhythm. The SCN receives direct photic input from the retina via the retinohypothalamic tract and sends timing signals to the rest of the brain and body. The pineal gland is downstream of the SCN: it receives SCN signals and secretes melatonin in darkness.
What is a zeitgeber?
A: The biochemical output signal of the SCN
B: An environmental time cue that synchronises ("entrains") the biological clock to the 24-hour day
C: A photoreceptor cell type responsible for circadian light detection
D: The period of a free-running circadian rhythm in the absence of time cues
Correct: An environmental time cue that synchronises ("entrains") the biological clock to the 24-hour day
Zeitgeber is German for "time-giver." It refers to any environmental cue that entrains the circadian clock — synchronising the slightly-off-24-hour free-running clock to the actual 24-hour day. Light (specifically short-wavelength/blue light in the ~480 nm range) is the primary zeitgeber for humans and is detected by specialised melanopsin-containing retinal ganglion cells. Secondary zeitgebers include meal timing, physical activity, social interaction, and temperature. Without zeitgebers, the human clock free-runs with a period slightly longer than 24 hours (~24.2 h).
Which of the following statements about melatonin is CORRECT?
A: Melatonin is secreted by the hypothalamus and promotes wakefulness during the day
B: Melatonin is secreted by the pineal gland in darkness, signalling night to the brain and body
C: Melatonin synthesis is suppressed by darkness and stimulated by light
D: Melatonin directly drives the molecular clock in SCN neurons
Correct: Melatonin is secreted by the pineal gland in darkness, signalling night to the brain and body
Melatonin is synthesised from serotonin in the pineal gland and released into the bloodstream. Its secretion is gated by the SCN: during darkness, the SCN disinhibits the superior cervical ganglion → noradrenaline → pinealocytes → melatonin release. Light — detected by melanopsin-containing ipRGCs in the retina — suppresses melatonin via the retinohypothalamic tract to the SCN. Melatonin is often called the "hormone of darkness": it signals to the body that it is night and helps regulate sleep onset, seasonal reproductive cycles, and immune function. It does not directly drive the molecular clock but feeds back to modulate SCN activity via MT1/MT2 receptors.
Which photoreceptor type is most important for the non-image-forming, circadian light response?
A: Rods (scotopic vision, rhodopsin)
B: S-cones (short-wavelength colour vision)
C: Intrinsically photosensitive retinal ganglion cells (ipRGCs) containing melanopsin
D: Müller glial cells that amplify light signals to the optic nerve
Correct: Intrinsically photosensitive retinal ganglion cells (ipRGCs) containing melanopsin
Intrinsically photosensitive retinal ganglion cells (ipRGCs) — discovered in the early 2000s by Ignacio Provencio and colleagues — express the photopigment melanopsin, which is maximally sensitive to short-wavelength (blue, ~480 nm) light. Unlike rods and cones (which adapt rapidly to sustained light), ipRGCs respond tonically to sustained illumination and project directly to the SCN via the retinohypothalamic tract. They are responsible for circadian entrainment, pupillary light reflex, and melatonin suppression. This explains why blue-light-emitting screens in the evening are particularly disruptive to circadian timing.
What is the biological mechanism of jet lag?
A: Dehydration and low cabin pressure during air travel suppress melatonin production
B: A mismatch between the traveller's internal circadian clock and the external light/dark cycle in the new time zone
C: Radiation exposure at altitude disrupts the CLOCK gene expression in the SCN
D: Disruption of sleep architecture caused by aircraft noise and uncomfortable seating
Correct: A mismatch between the traveller's internal circadian clock and the external light/dark cycle in the new time zone
Jet lag occurs when rapid travel across multiple time zones creates a mismatch between the traveller's internal circadian clock (which remains set to the departure time zone) and the external light–dark cycle and social schedule of the destination. The SCN and peripheral organ clocks re-entrain to the new time zone at different rates (~1 day per time zone crossed) and may re-synchronise at different speeds relative to each other — causing internal desynchrony. Symptoms include sleep disturbance, daytime fatigue, cognitive impairment, gastrointestinal disturbance, and mood changes. Eastward travel is generally harder to adjust to (requiring phase advance) than westward travel (phase delay).
What is a chronotype, and which developmental period is characterised by a systematic shift toward eveningness?
A: The speed at which the circadian clock entrains to a new time zone; middle age shows slowest entrainment
B: An individual's biological tendency toward preferred sleep/wake timing (morning vs evening); adolescence is characterised by a shift toward eveningness
C: The duration of the free-running period of the SCN; it does not change across development
D: A subtype of circadian rhythm disorder; it is most common in the elderly due to advanced sleep phase
Correct: An individual's biological tendency toward preferred sleep/wake timing (morning vs evening); adolescence is characterised by a shift toward eveningness
Chronotype refers to an individual's biological preference for timing of sleep and wakefulness — the "morning lark" vs "night owl" dimension. Chronotypes are partly heritable (~50%) with contributions from variants in clock genes (PER3, CLOCK). Chronotype shifts systematically across development: children are relatively early types, adolescents shift dramatically toward eveningness (peaking around age 19.5 in females and 21 in males in Till Roenneberg's data), then gradually shift back toward morningness in adulthood, with older adults becoming early types. Adolescents' evening preference means that early school start times conflict with their biological timing, contributing to sleep deprivation.
The 2017 Nobel Prize in Physiology or Medicine was awarded for the discovery of molecular mechanisms controlling circadian rhythms. Who received it?
A: Nathaniel Kleitman and Eugene Aserinsky (for discovering REM sleep)
B: Jeffrey Hall, Michael Rosbash, and Michael Young (for cloning and characterising the period and timeless genes in Drosophila)
C: Joseph Takahashi and Michael Menaker (for identifying the tau mutation and the CLOCK gene)
D: Ignacio Provencio and Russell Foster (for discovering melanopsin and ipRGCs)
Correct: Jeffrey Hall, Michael Rosbash, and Michael Young (for cloning and characterising the period and timeless genes in Drosophila)
Jeffrey Hall, Michael Rosbash, and Michael Young received the 2017 Nobel Prize for elucidating the molecular clock mechanism. Working in Drosophila, they cloned the period (per) gene (first identified by Ron Konopka and Seymour Benzer in 1971), described the protein it encodes (PER), discovered its accumulation and degradation over 24 hours, and identified timeless (tim) as an interacting partner. Their work established the core feedback loop: CLOCK–BMAL1 proteins activate transcription of PER and CRY; PER and CRY proteins accumulate, then inhibit CLOCK–BMAL1, suppressing their own transcription. This ~24-hour loop is conserved across organisms from fruit flies to humans.
Which health outcomes are associated with chronic circadian disruption from long-term shift work?
A: Improved metabolic efficiency; shift workers tend to have lower BMI due to higher activity at night
B: Increased risks of metabolic syndrome, type 2 diabetes, cardiovascular disease, certain cancers, and depression
C: Primarily psychological effects (depression, anxiety) with no physiological consequences
D: Stronger immune function due to sustained activation of the SAM axis
Correct: Increased risks of metabolic syndrome, type 2 diabetes, cardiovascular disease, certain cancers, and depression
Decades of epidemiological research on shift workers (nurses, factory workers, long-haul truckers) consistently shows elevated risks of: metabolic syndrome and type 2 diabetes (circadian disruption impairs glucose metabolism and insulin sensitivity); cardiovascular disease (dysregulated blood pressure circadian pattern, inflammation); breast and colorectal cancer (the International Agency for Research on Cancer classifies shift work as a probable carcinogen based on melatonin disruption and immune effects); mood disorders; and gastrointestinal disorders. The mechanisms involve misalignment between the central SCN clock and peripheral organ clocks (liver, pancreas, adipose tissue) that disrupts the temporal organisation of metabolism, immune function, and cell cycle regulation.
Circadian Rhythms
Where is the master circadian pacemaker located in the human brain?
About this quiz
The word "circadian" comes from the Latin circa diem — approximately a day. Every cell in your body runs on a roughly 24-hour molecular clock, orchestrated by a master pacemaker in the brain and synchronised to the outside world by light.
This quiz covers the mechanisms of the biological clock — from the suprachiasmatic nucleus and melatonin to chronotypes, jet lag, and the health consequences of circadian disruption.