Circadian Rhythms and Chronobiology

• Circadian Rhythms: Biological processes that display an endogenous, built-in rhythm (about 24 hours) influenced by external cues (zeitgebers).

• Chronobiology: Study of biological clocks and associated rhythms that control the timing of physiological and behavioral events in organisms.

• Chronotype: Individual differences in the timing of sleep and activity patterns; includes early risers (larks) and night owls.

Types of Biological Rhythms

• Ultradian Rhythms: Cycle lasts less than 24 hours (e.g., stages of sleep).
• Circadian Rhythms: Cycle lasts about 24 hours (e.g., sleep-wake cycles).
• Circannual Rhythms: Cycle lasts about a year (e.g., seasonal breeding in animals).

Sources of Biological Rhythms

• Exogenous Factors: External factors like light, dark, and behavioral patterns affecting sleep/wake cycles.
• Endogenous Factors: Internal biological clocks that regulate physiological and behavioral rhythms.

First Evidence of Biological Rhythms

• 1729 Study: Jean Jacques de Mairan showed heliotropic plants continued to open and close with day-night cycle even in the absence of light, indicating an internal biological rhythm.

Measuring Biological Rhythms

• Movement Records: Activity logs (actograms) used to track sleep/wake cycles. Each day is represented as a row on a vertical graph, with time on the x-axis.
• Components of Rhythms: Includes maximum, minimum, amplitude (change above/below average), period length ( au), and frequency (number of cycles per time unit).

Circadian Clocks

• Location: Master clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Peripheral clocks located throughout the body.
• Lesion Studies: Bilateral lesions in SCN eliminate circadian rhythms, confirming its role as the master clock.
• Transplant Studies: SCN tissue transplants can change the circadian rhythm of the recipient, showing functionality of the SCN.

Zeitgebers

• Definition: External cues (time-givers) that help synchronize internal biological clocks.
• Examples: Light is the primary zeitgeber, but social cues, temperature, and meal timings also play roles.

Characteristics of Circadian Rhythms

• Must have a cycle of about 24 hours.
• Must persist in absence of zeitgebers (endogenous).
• Must be trainable by external cues (entrainable).

Jet Lag and Phase Shifts

• Jet Lag: A specific phase shift occurring when travelling across time zones, leading to misalignment between internal biological clock and local time.
  • Easier West: Phase delays (traveling west) are generally easier than phase advances (traveling east).
• Phase Response Curve: Relationship between the timing of an external cue (light event) and its effect on circadian rhythms.

Hormonal Regulation

• Melatonin: Produced by the pineal gland, regulates sleep cycles; production is inhibited by light, particularly blue light (460-480 nm).
  • Light exposure suppresses melatonin secretion, while darkness stimulates it.
• Glucocorticoids: Have a strong circadian rhythm regulated by the SCN, persisting even under constant light.

Sleep Disorders Related to Circadian Rhythms

• Advanced Sleep-Phase Syndrome (ASPS): Circadian clock runs too fast; requires treatment via light therapy.
• Shift-Worker Syndrome: Results from irregular sleep patterns due to external conditions; treated with bright lights during work shifts.

Seasonal Affective Disorder (SAD)

• Description: Seasonal depression characterized by lethargy, anxiety, and impaired concentration during autumn/winter months.
• Causes and Treatments:
  • Linked to changes in serotonin levels as related to light exposure.
  • Light therapy effective at treating symptoms by re-aligning circadian rhythms.

Guiding Questions for Understanding

• What defines chronobiology and chronotypes?
• How do external and internal factors shape circadian rhythms?
• What is the function of the SCN as the master clock?
• How do zeitgebers differ from our internal clocks?
• What treatments exist for circadian rhythm-related disorders?