How are biological rhythms classified by period length, and which category has a period shorter than 24 hours?
A: Circadian (>24h), ultradian (=24h), infradian (<24h)
B: Ultradian (<24h), circadian (~24h), infradian (>24h)
C: Infradian (<24h), circadian (~24h), ultradian (>24h)
D: All biological rhythms are circadian; the others are pathological deviations
Correct: Ultradian (<24h), circadian (~24h), infradian (>24h)
Biological rhythms are classified by the length of their cycle (period): ultradian rhythms have a period shorter than 24 hours and repeat multiple times per day (examples: heartbeat, breathing, hunger waves, 90-minute sleep/rest cycles, pulsatile hormone secretion). Circadian rhythms have a period of approximately 24 hours and are entrained by light–dark cycles. Infradian rhythms have a period longer than 24 hours (examples: weekly appetite cycles, the ~28-day menstrual cycle, seasonal/annual rhythms). The prefixes come from Latin: ultra = beyond, infra = below, circa = approximately, dies = day.
What is the Basic Rest-Activity Cycle (BRAC), and who first described it?
A: A 24-hour cycle of metabolic activity and rest discovered by Hans Selye during his stress research
B: A ~90-minute cycle of alternating physiological arousal and quiescence that operates during both sleep and waking, proposed by Nathaniel Kleitman
C: A weekly rhythm of work and rest whose disruption causes burnout, described by Maslach
D: The alternating activation of sympathetic and parasympathetic nervous systems over 60-minute cycles during the day
Correct: A ~90-minute cycle of alternating physiological arousal and quiescence that operates during both sleep and waking, proposed by Nathaniel Kleitman
Nathaniel Kleitman (who also co-discovered REM sleep) proposed in 1963 that the ~90-minute sleep cycle — the period between successive REM episodes — reflects a fundamental ultradian "basic rest-activity cycle" (BRAC) that continues as a waking rhythm. During waking, periods of relative alertness and focused attention (~80–90 min) are followed by brief troughs of drowsiness, daydreaming, and reduced cognitive performance (~10–20 min). Evidence includes: 90-minute cycles in gastric motility, nasal airflow lateralisation (nasal cycle), fantasising/daydreaming, and oral activity. Proponents suggest that taking short breaks every ~90 minutes aligns with this natural ultradian cycle.
Pulsatile secretion of hormones is a common ultradian rhythm. Which example best illustrates this?
A: Cortisol peaks once per day in the morning (cortisol awakening response) — a circadian, not ultradian rhythm
B: Growth hormone (GH) is secreted in large pulses every 3–5 hours, with the largest pulse occurring during early SWS
C: Melatonin is secreted in hourly pulses throughout the night, peaking around 3 AM
D: LH (luteinising hormone) is secreted in a single daily pulse that coordinates the menstrual cycle
Correct: Growth hormone (GH) is secreted in large pulses every 3–5 hours, with the largest pulse occurring during early SWS
Growth hormone (GH) release is a clear example of ultradian hormone secretion: it is released in discrete pulses every 3–5 hours, with the largest and most consistent pulse occurring during the first N3 (slow-wave sleep) episode of the night. Pulsatile GH secretion is critical because GH receptors down-regulate with continuous exposure — only pulsatile delivery maintains receptor sensitivity. Other examples of ultradian hormone pulses include: gonadotropin-releasing hormone (GnRH) from the hypothalamus every ~60–120 min (driving LH/FSH pulses); insulin secretion in oscillations every 10–15 minutes; and cortisol pulses every 60–90 minutes superimposed on its broader circadian rhythm.
The menstrual cycle is the best-known infradian rhythm in humans. Which hormonal change triggers ovulation at approximately day 14?
A: A sharp rise in progesterone from the developing corpus luteum
B: A surge in oestrogen from the dominant follicle, triggering a surge in luteinising hormone (LH)
C: A rise in cortisol in response to follicular stress that stimulates the hypothalamus
D: A fall in oestrogen and progesterone that removes negative feedback and allows FSH to peak
Correct: A surge in oestrogen from the dominant follicle, triggering a surge in luteinising hormone (LH)
The menstrual cycle is regulated by a hormonal cascade: FSH stimulates follicular growth → growing follicles produce increasing oestrogen → at a threshold concentration, oestrogen switches from negative to positive feedback on the hypothalamus–pituitary axis → a massive LH surge (and smaller FSH surge) → ovulation within ~36 hours. Post-ovulation, the ruptured follicle becomes the corpus luteum and produces progesterone (and oestrogen), supporting potential implantation. If no implantation occurs, the corpus luteum degenerates, progesterone and oestrogen fall, and menstruation occurs. The cycle is typically ~28 days but varies between individuals (21–35 days) and within the same individual.
Seasonal Affective Disorder (SAD) is a clinical example of disrupted infradian rhythm. What is its proposed biological mechanism?
A: Excessive cortisol production in winter caused by reduced physical activity and increased indoor time
B: Phase-delayed circadian clock in winter combined with altered melatonin secretion and reduced serotonin transmission due to shorter photoperiod
C: Vitamin D deficiency in winter that directly inhibits dopamine synthesis in the ventral striatum
D: Dysregulation of the hypothalamic–pituitary–thyroid axis in response to cold temperatures
Correct: Phase-delayed circadian clock in winter combined with altered melatonin secretion and reduced serotonin transmission due to shorter photoperiod
Seasonal Affective Disorder (SAD) — recurrent depression with a seasonal pattern, typically autumn/winter onset and spring remission — is thought to involve: (1) a phase delay of the circadian clock in winter, when later sunrise prolongs morning darkness; (2) prolonged melatonin secretion (the "melatonin hypothesis"); and (3) reduced serotonin transporter efficiency and lower synaptic serotonin in winter, potentially driven by reduced light-driven serotonin synthesis. Light therapy (10,000 lux bright light for 30 min each morning) is the most evidence-based treatment — working by phase-advancing the circadian clock, suppressing morning melatonin, and stimulating serotonin synthesis. This directly targets the circadian/seasonal desynchrony rather than the depression secondarily.
Which of the following best demonstrates that infradian rhythms have functional biological significance beyond simply counting days?
A: Women who live together show no menstrual synchrony, demonstrating rhythms are independent of social influences
B: Reproductive hormones cycle across the menstrual cycle and produce systematic changes in cognition, mood, sexual motivation, and immune function across the cycle
C: Infradian rhythms are primarily cultural constructs; the menstrual cycle length varies so widely across populations that it lacks a consistent biological basis
D: Only non-human animals show true infradian rhythms; human annual cycles are entirely driven by cultural factors like calendar awareness
Correct: Reproductive hormones cycle across the menstrual cycle and produce systematic changes in cognition, mood, sexual motivation, and immune function across the cycle
Infradian rhythms are functionally significant: hormonal fluctuations across the menstrual cycle produce measurable effects on mood (premenstrual syndrome/PMDD in susceptible individuals), cognitive performance (some studies show verbal fluency advantages in high-oestrogen phases, spatial ability in low-oestrogen phases), sexual motivation (peaks around ovulation), pain sensitivity, and immune function (progesterone is immunomodulatory). Circannual rhythms (annual biological cycles) also occur in humans: seasonal variation in birth rates, testosterone levels, mood, appetite (carbohydrate craving in winter), and immune markers are all documented. These observations indicate that infradian rhythms are embedded in physiology, not merely behavioural patterns.
Ultradian & Infradian Rhythms
How are biological rhythms classified by period length, and which category has a period shorter than 24 hours?
About this quiz
Not all biological rhythms run on a 24-hour cycle. Ultradian rhythms cycle faster than a day — the 90-minute sleep cycle, hunger waves, hormone pulses. Infradian rhythms cycle slower — the menstrual cycle, seasonal mood changes, annual migratory urges.
This quiz covers the definition, mechanisms, and examples of ultradian and infradian rhythms, and their psychological and health implications.