1. CIRCADIAN CLOCK RYTHMS | 24 - HOUR BIOLOGICAL BODY CLOCK | SECRETS OF PHYSICS | RABIA BABER
Circadian Rhythm and Circadian Clock
Circadian clocks are internal biological timing systems that regulate physical, mental, and behavioral changes over a roughly 24-hour cycle. These clocks help organisms know when to sleep, wake, eat, release hormones, repair cells, and perform metabolic activities.
The word circadian comes from Latin:
• circa = “around”
• diem = “day”
So circadian means “around a day.”
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Discovery of Circadian Clocks
Early Observations (1729)
The first known circadian experiment was conducted by French scientist:
Jean-Jacques d'Ortous de Mairan
He observed that mimosa plant leaves opened during the day and closed at night. Even when kept in complete darkness, the leaves still followed the same daily rhythm.
This showed that living organisms possess an internal clock independent of sunlight.
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20th Century Discoveries
Scientists later discovered biological timing systems in animals and humans.
Important contributors include:
• Colin Pittendrigh — often called the “father of circadian biology”
• Jeffrey C. Hall
• Michael Rosbash
• Michael W. Young
These scientists discovered genes controlling circadian rhythms, including the famous:
• PER (period) gene
• CLOCK gene
• BMAL1 gene
Their discoveries earned the 2017 Nobel Prize in Physiology or Medicine.
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The Master Clock: Suprachiasmatic Nucleus
The main circadian clock in humans is located in the brain inside the:
• SCN (Suprachiasmatic Nucleus)
• Found in the hypothalamus
• Contains about 20,000 neurons
The SCN receives light signals from the eyes and synchronizes the body with the day-night cycle.
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How Circadian Clocks Work
The circadian system uses:
• Light exposure
• Darkness
• Hormones
• Temperature
• Feeding schedules
to regulate timing.
Main hormones:
• Melatonin → promotes sleep
• Cortisol → promotes alertness
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What Different Circadian Timings Suggest
4 AM – 6 AM
Body Preparation Phase
• Body temperature starts rising
• Cortisol begins increasing
• Brain prepares for wakefulness
Suggests:
• Best time for early athletes and meditation practitioners
• Very low alertness if sleep-deprived
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6 AM – 9 AM
Natural Wake-Up Window
• Cortisol peaks
• Blood pressure rises
• Alertness increases
Suggests:
• Good time for waking naturally
• Morning sunlight strongly resets the circadian clock
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9 AM – 12 PM
Peak Cognitive Performance
• Brain alertness high
• Focus and reaction time improve
Suggests:
• Best time for studying
• Deep work and problem solving
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12 PM – 3 PM
Energy Dip
• Mild drop in alertness
• Digestive activity increases
Suggests:
• Post-lunch sleepiness is biologically normal
• Short naps may help
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3 PM – 6 PM
Physical Performance Peak
• Muscle strength highest
• Coordination improves
• Body temperature peaks
Suggests:
• Best time for exercise
• Strong athletic performance
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6 PM – 9 PM
Wind-Down Phase
• Melatonin begins increasing after darkness
• Alertness gradually decreases
Suggests:
• Reduce blue light exposure
• Ideal time for relaxing routines
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9 PM – 12 AM
Sleep Initiation Window
• Melatonin secretion high
• Body temperature decreases
Suggests:
• Best sleep quality usually begins here
• Late-night stimulation disrupts rhythm
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12 AM – 4 AM
Deep Recovery Phase
• Deep sleep and REM cycles dominate
• Growth hormone release increases
• Cellular repair active
Suggests:
• Critical period for brain repair and memory consolidation
• Night shift work can strongly disrupt health
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Circadian Rhythm and Health
Healthy circadian alignment supports:
• Better sleep
• Improved metabolism
• Hormonal balance
• Strong immune function
• Mental health
Circadian disruption is linked with:
• Obesity
• Diabetes
• Depression
• Cardiovascular disease
• Some cancers
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