Anatomy of Circadian Rhythms in the Body

Ashley Davidoff MD Copyright 2015

To every thing there is a season, and a time to every purpose under the heaven:

 A time to be born, and a time to die; a time to plant, and a time to pluck up that which is planted;

Ecclesiastes 3:1

What is a season?

A season in the classical sense, is a natural division of the year and relates to the position of the earth to the sun.  The sun is a source of light, warmth, and energy.  A season is a continuum of time divided by the rhythmicity of four seasons.   The seasons are characterized by changes in light and dark cycles as well as temperature cycles.  These factors impact life in all its aspects.

Cycles Around and Within Us

This image shows the  seasonal cycle that occurs within a year.  The top left image is the spring when rebirth and restoration of life occurs.  The summer (top right) follows where light and temperature are high.  The mature mellow autumn follows with orange reds and yellow colors (bottom right) .  Light starts to fade and temperatures start to fall  Autumn is a time for introspection and recognition of mortality.  The bleak and cold winter follows with a silence of impending death (bottom left).  The dancers with their own cycles are incorporated into the larger cycle, inferring the intimate integration of internal biology with the changes in the environment.  They are positioned like the hands of a clock – again incorporating the element of time.

• Spring is a time for birth, freshness and renewal
• Summer brings energy, blossoming, productivity and maturation and is the prime of life. Light is brightest during this period and the weather is warmest
• Fall brings mellowness and maturation and warns of decline.  Symbolically it is a time for introspection during mid life
• Winter is the time of aging, bleakness, decline and death.  It is characterized by darkness and cold and symbolizes quietness, contemplation, and reminds us of our  impending old age and death.

“To every thing  there is a season”  What does that mean to the structures of the body?  

The changes of the seasons have parallels in the internal environment of the body down to the cells and molecules.  This implies that “everything” in biology experiences a season.  

The annual cycle of the seasons is experienced physically by the body, its organs, cells, and molecules throughout the year.  They sense the cold and the warmth, and the light and dark of the external environment.

Seasons of the Feet

The AiA rendering is a collage of photographs with superimposed CT scan of the feet as they walk through the seasons.  The four seasons created in a sphere provide a sense of wholeness in time, while the footsteps represent a walk in  time.  The top left image of the sphere shows the magnolias of Spring, the next (going clockwise) is green forest of Summer, followed by the Fall and finally the deathly cold of winter.

Seasons of the Abdomen

This AiA rendering shows the abdominal structures reflecting the seasons.  The spring is in shades of  pink, the summer in green of the forest and the blue of the sky, the fall in oranges and red, and the winter in ice blue and white.

Seasons within Seasons and Cycles within Cycles

The cycles of the body range from nano seconds per cycle to a lifetime.  Each cycle is part of another cycle.  Each cycle consists of a beginning that evolves to full speed production, progressing to a winding down phase and  finally a phase of rest and restoration. The Krebs cycle,  heart cycle, circadian cycle, menstrual cycle, are all part of the wheel within a wheel concept and in the grand scheme of a single life it revolves around the process  of  maturation.  These same cycles occur within the cycles of evolution with improvement in processes over time.

Biochemical cycles are responsible for growth, and reproduction. The clock has to be rewound, restored to step one so that it can do its duty again. All of these have been optimized by the grand Darwinian cycle of reproduction, generation after generation, picking up fortuitous improvements over the eons.

Seasons of the Day –  Parallel Changes in the External and Internal Environments

Seasons of the Body in a 24 hour Period

This image shows the parallel of a 24 hour cycle correlating with the seasonal cycle that occurs within a year.  The top left image is the dawn when the day is born and the person is restored from sleep and starts to get ready for the the next 12 hour period.  The stress hormones start to rise as the body prepares for the work day which reflects the summer time (top right) where light and temperature are high.  After the “summer” the mellow and introspective time of the sunset arrives(bottom right)  as light and temperature start to fade.  The temporary death of sleep provided by the night follows in the bottom left .

Daybreak at sunrise is the birth of the day, and represents spring time in the body.  We wake up restored while it is a little light and a little cold outside and are refreshed and reborn after our slumber.  As the day warms up, and the light becomes stronger, we enter the summer of our 24 hour cycle.  Our cortisol levels are up to deal with the physical stresses of the day and this preparation is felt in our cells and in our psyche.  As a result of the stress we are better able to process and produce at work.  During the day we reach the height of our wakefulness and are in our productive prime.    At dusk, the autumn of the day moves in as we start to wind down and become introspective.  The light starts to go down and without the sun, the day cools down. The winter of the day – nighttime arrives. It is colder and darker, and we prepare for sleep, a transient death that helps us restore.

The internal environment of the body also senses the cycles through a hormonal cycle called the circadian cycle, which is executed via the hypothalamus.   enabling rhythmic physiological and behavioral events of the body to work in parallel with the natural rhythms of the external environment.

“Anatomy of Sleep” 

The central control system for the circadian rhythms is in the suprachiasmatic nucleus (SCN) of the hypothalamus (turquiose blue butterfly shaped structure at the base of the brain)  It is responsible for the control of the circadian rhythms of the body.

Seasons of the Molecules

Circadian Rhythm and the Daily Internal Seasons of the Body 

There are three main physiological events in the daily human diurnal circadian rhythm; melatonin secretion, cortisol secretion, and temperature variation.

Melatonin

Melatonin is secreted by the pineal body in response to the dark.  The retina identifies light and transmits this information to the suprachiasmatic nucleus (SCN) in the hypothalamus. The hypothalamus integrates the light and darkness cycles of the environment with the pineal gland which in turn secretes melatonin in the dark.  Melatonin levels are therefore high at night and low in the day.  Melatonin is involved in sleep cycles, blood pressure regulation, and in seasonal reproduction.

Cortisol

Seasons at the Molecular Level in the Adrenal Gland

The adrenal gland is an essential component of the circadian rhythm.  It produces cortisol which prepares the body for the stresses of the day. The artistic rendering shows background of complex biochemistry artistically incorporated into the gland. This is a featured art piece  because of the beautiful manner in which the almost invisible gland transforms the biochemistry into life giving hormones.

Cortisol is a hormone produced by the adrenal gland that is elemental in the control of metabolic function. It prepares the body for impending stresses.  Diurnal secretion of cortisol is part of circadian rhythm physiology.  The level is highest in the early morning and is at its lowest level at about midnight to 4 am. This high level in the morning prepares the body for the stress of the day.  Cortisol is intimately involved in the metabolism of glucose, fat and protein.

Body Temperature

Body temperature is lowest about 2 hours before waking in the morning and highest in the late afternoon and early afternoon.  During the night metabolic activity is slowed and the lowered temperature is a mechanism to conserve energy.  temperature is elemental to all biochemical processes.

“Body Temp Variation” by user:RHorning – Image Source. Licensed under Public Domain via Commons –

Seasons of the Cells

The molecular changes give rise to cycles in the cells both in a minute to minute variations, hour to hour variations, and eventually into annual variations

Seasons in the Squamous Flower

Cycles in the Columnar Cell of the Colon

What would life be like if there were no seasons?

Although there is a pattern to the seasons – each one is different and as we gain perspective on the cycles.

There are cycles within cycles as time moves on – each not the same as the prior, but having many things in common.  The beginning of Spring is so full of life and we soon look forward and want to welcome the blooming of the brighter colors of summer.  And then the heat of summer comes and we look forward to the cool and mature fall – and the beautiful first deep snowfall comes and soon grows old, cold, uncomfortable and dreary – and the new cycle begins with our eyes a little changed

Eachseasons bring a lesson of life.  We need the seasons to feel the rhythm within us and around us… but each year the seasons are different, and we learn and grow with each season.

The dawn brings us newness and freshness.  The day brings us energy in full flight and productivity – the dusk brings quietness and satisfying mellowness and the night brings rest and restoration….   But each day is different, and hopefully we learn and grow with each day

“Life Cycle of the Uterus”

shows the uterus from birth through maturation to senility 

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