13.3 Circadian rhythms and biological clocks

Circadian rhythms are internal body clocks that regulate our daily cycles. These 24-hour patterns control sleep, hormone release, and other vital functions. They're influenced by genes and environmental cues like light.

The suprachiasmatic nucleus in our brain acts as the master clock, coordinating other body clocks. Melatonin, a hormone released at night, helps regulate sleep. Disrupting these rhythms can lead to health issues and sleep problems.

Circadian Rhythm and Biological Clocks

Endogenous Rhythms and Clock Genes

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• Circadian rhythms are endogenous, roughly 24-hour cycles that persist even in the absence of external cues
• Free-running rhythms occur when an organism is isolated from environmental cues and continues to follow an approximately 24-hour cycle
• Clock genes, such as

  Period

  and

  Timeless

  in Drosophila, regulate the expression of other genes and control the timing of physiological processes
• Clock genes form a negative feedback loop that oscillates with a period of about 24 hours (example:

  CLOCK

  and

  BMAL1

  in mammals)

Entrainment and Synchronization

• Entrainment is the process by which circadian rhythms are synchronized to the 24-hour day-night cycle
• Environmental cues, such as light and temperature, can reset the biological clock and adjust the phase of the circadian rhythm
• Jet lag occurs when the circadian rhythm is out of sync with the local time due to rapid travel across time zones
• Shift work can disrupt the circadian rhythm and lead to sleep disorders and other health problems

Suprachiasmatic Nucleus (SCN) and Melatonin

The Master Clock

• The suprachiasmatic nucleus (SCN) is a small region in the hypothalamus that acts as the master pacemaker of the circadian system
• The SCN receives input from the retina about light levels and synchronizes peripheral clocks throughout the body
• Lesions to the SCN can disrupt circadian rhythms and lead to arrhythmic behavior
• The SCN controls the release of melatonin from the pineal gland

Melatonin and Sleep Regulation

• Melatonin is a hormone secreted by the pineal gland that regulates sleep-wake cycles
• Melatonin levels rise in the evening, promoting sleep, and fall in the morning, promoting wakefulness
• Exposure to light at night can suppress melatonin production and disrupt sleep (example: blue light from electronic devices)
• Melatonin supplements can be used to treat sleep disorders and jet lag

Phase Shifts and Circadian Disruption

• Phase shifts occur when the circadian rhythm is advanced or delayed relative to the external environment
• Light exposure at certain times can cause phase advances (earlier sleep and wake times) or phase delays (later sleep and wake times)
• Chronic circadian disruption, such as from shift work or irregular sleep schedules, can lead to metabolic disorders, cardiovascular disease, and cancer
• Strategies to maintain a healthy circadian rhythm include regular sleep schedules, avoiding light at night, and getting exposure to natural light during the day

Environmental Cues and Synchronization

Zeitgebers and Entrainment

• Zeitgebers are environmental cues that synchronize the circadian rhythm to the 24-hour day
• Light is the most potent zeitgeber, but other cues such as temperature, food availability, and social interactions can also entrain the circadian clock
• The phase response curve describes how the timing of light exposure affects the phase of the circadian rhythm
• Gradual changes in zeitgebers, such as the seasonal changes in day length, can cause the circadian rhythm to adapt over time

Photoperiod and Seasonal Rhythms

• Photoperiod is the length of the light phase in a 24-hour cycle
• Many organisms use changes in photoperiod to synchronize seasonal rhythms, such as reproduction, migration, and hibernation
• Long-day plants flower when the day length exceeds a critical threshold, while short-day plants flower when the day length falls below a critical threshold
• Animals such as birds and mammals use changes in photoperiod to regulate breeding seasons and prepare for winter (example: molting in birds, antler growth in deer)