Circadian Rhythms
• 1729: French Astronomer Mairan reports that the mimosa plant closes its leaves at night even when kept in
constant darkness
• In other words, they have an internal clock, and do not simply react to the changes in lighting levels
• Latin: circa = approximately, dies = day
• Found in all kingdoms of living organisms
• Internally generated
• Continuously adjusted, entrained, mainly by the external light: dark cycle
• Will persist even without external time cues
• Environment is not constant
• Important, daily changes in environmental factors are predictable:
i. Light
ii. Temperature
iii. Food availability
• Circadian rhythms allow us to anticipate (as opposed to passively, and belatedly, reacting to) these predictable changes
• Sleep-wake rhythm in a human volunteer with and without an external light-dark cycle
• Intrinsically generated circadian rhythms allow us to anticipate (rather than react) to changes across the 24-hour
cycle
1. HORMONES 3. PERFORMANCE
• Melatonin (night) • Alertness (late morning)
• Cortisol (morning)
4. SLEEP
2. PHYSIOLOGY • Night
• Bowel movements (mornings)
• Glomerular filtration (day)
• Blood pressure (early evening)
• Body temperature (early evening
• Cortisol
• Growth hormone
• Sex hormones
• Melatonin
Integration of Physiological Systems Page 1
• 1729: French Astronomer Mairan reports that the mimosa plant closes its leaves at night even when kept in
constant darkness
• In other words, they have an internal clock, and do not simply react to the changes in lighting levels
• Latin: circa = approximately, dies = day
• Found in all kingdoms of living organisms
• Internally generated
• Continuously adjusted, entrained, mainly by the external light: dark cycle
• Will persist even without external time cues
• Environment is not constant
• Important, daily changes in environmental factors are predictable:
i. Light
ii. Temperature
iii. Food availability
• Circadian rhythms allow us to anticipate (as opposed to passively, and belatedly, reacting to) these predictable changes
• Sleep-wake rhythm in a human volunteer with and without an external light-dark cycle
• Intrinsically generated circadian rhythms allow us to anticipate (rather than react) to changes across the 24-hour
cycle
1. HORMONES 3. PERFORMANCE
• Melatonin (night) • Alertness (late morning)
• Cortisol (morning)
4. SLEEP
2. PHYSIOLOGY • Night
• Bowel movements (mornings)
• Glomerular filtration (day)
• Blood pressure (early evening)
• Body temperature (early evening
• Cortisol
• Growth hormone
• Sex hormones
• Melatonin
Integration of Physiological Systems Page 1
