The Fight-or-Flight Response & The Circadian Rhythm

Stress is a natural and essential reaction in the body, where the brain responds to new situations - enabling us to quickly adapt and solve physical and mental challenges.

Chronic stress, however, is an unwanted state where the brain continuously believes that we are under threat, and the body is constantly prepared for a fight. This places a huge strain on our bodies, and is known to be a major factor leading to many physical and mental illnesses.

The Autonomic Nervous System.

The autonomic nervous system is a control system that automatically and unconsciously regulates many bodily functions, including our reactions to stress.

It comprises two complementary parts: the sympathetic nervous system (the ‘fight-or-flight' system) and the parasympathetic nervous system (the ‘rest and digest’ system). These two systems typically work in opposition to one another to achieve homeostasis - the state of ‘steady’ conditions for optimal functioning of an organism. A useful analogy would be that the sympathetic system works as an accelerator, and the parasympathetic system is the brake.

The sympathetic system prepares our body for action and becomes especially active in stressful situations and during hard physical effort. When faced with a threatening or rapidly escalating situation, the fight-or-flight response is automatically triggered - even before we have time to consciously decide to act.

When this happens, the hormones epinephrine (also known as adrenaline) and norepinephrine are released into our system. These have an almost instantaneous effect - increasing our blood pressure, pulse rate, and oxygen intake - making us more alert, stronger, and quicker. Our senses become sharper, glucose and fats are released into the bloodstream to give us instant energy and power, and the palms of our hands sweat - to give us improved grip should we need to climb a tree.

Simultaneously, the parasympathetic system shuts down, and our recovery and healing systems, immune system, and food digestion and processing systems are turned off - to conserve all available energy for the muscles and the senses.

If the perceived threat turns out not to be a danger, the parasympathetic system becomes active again and returns the body back to its normal relaxed state. If, however, the threat is real and a fight seems unavoidable, the hypothalamic-pituitary-adrenal axis in the brain is activated, and the steroid hormone cortisol is released - which keeps the sympathetic nervous system running for as long as it is needed.

When considering our ancient origins, it must be remembered that although our ancestors evolved to face threats daily and fight for their lives, these were normally situations that escalated rapidly and were over quickly - and so their exposure to prolonged episodes of stress was limited.

Today, our stress does not normally come from being chased by a wild animal or attacked by a neighbouring tribe, but is an accumulation of long-term worry, fear, and emotional and cognitive load - which are part of everyday modern life. These can compound and multiply until they result in a situation where the body is in a permanent fight-or-flight state - known as chronic stress.

Chronic Stress.

In chronic stress, the hypothalamic-pituitary-adrenal axis remains constantly active, pumping stress hormones, such as cortisol, into our systems - depressing the activity of the parasympathetic nervous system and causing severe disturbance to our vital metabolic mechanisms.

The constant levels of cortisol and other stress hormones compromise our immune systems, which can lead to infections and the development of autoimmune diseases and cancer. The continuous boosts of adrenaline raise blood pressure, harming blood vessels and increasing the risk of cardiovascular disease.

Cortisol tells our body to have a ready supply of energy available. So it increases our appetite (especially for calorie-dense foods) and promotes energy storage as fat - leading to weight gain. It also gets released into the hippocampus in our brain, affecting a person's ability to concentrate and regulate their emotions.

Eventually, a prolonged period of chronic stress will cause burnout, and can result in severe physical and psychological consequences.

Stress and the Circadian Rhythm

Circadian Rhythms are 24-hour cycles that are part of the body’s internal clock. They are synchronised with the ‘master clock’ (the SCN Nucleus) within the hypothalamus in our brain.

The SCN is extremely sensitive to light, and so our circadian rhythms are closely connected to day and night, being reset by sunlight in the morning - although other cues such as exercise, illness, environmental conditions, and temperature can also have an influence.

There are biological clocks in nearly every tissue and organ in our body, which this Master Clock in our brain controls, and these influence essential functions within our bodies at specific times on the 24-hour cycle.

This master clock also regulates the baseline rhythms of the autonomic nervous system. From its peak activity in the morning, throughout the later part of the day the sympathetic nervous system is slowly shut down, and the parasympathetic system is activated. This moves our bodies from a fight-or-flight readiness during the day into rest and repair mode as we sleep.

During this time, numerous essential self-healing, recycling, waste removal, and regeneration processes take place, and our immune system becomes more active - which is why colds often appear to get worse during the late evening and night time, due to the inflammation caused as our bodies fight any infections present.

However, when your body is in the condition of chronic stress - because the hypothalamic-pituitary-adrenal axis remains constantly activated, it overrides the circadian rhythm, keeping the sympathetic nervous system active for longer and stalling the activation of the parasympathetic system.

This can have severe physical and psychological repercussions, as the body and mind cannot fully regenerate and prepare for the oncoming day.

To maximise the self-healing capabilities of the body, and bring balance and order to our day, it is essential to align our behaviours with our Circadian Rhythm. We can actively achieve this by;

• Reducing our stress levels
• Getting early morning exposure to natural light
• Following a consistent eating, sleeping, and waking schedule
• Keeping active throughout the day
• Improving sleep hygiene - by making the bedroom conducive to deep sleep.