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 in our brain acts as the master clock, coordinating other body clocks. , 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
, 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 )
Entrainment and Synchronization
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
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
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)