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Why We Fall Back Into Old Habits When We’re Tired or Stressed

It has taken millions of years for the human brain to reach its current state.

  • The lizard brain controls our vital functions, such as breathing, digestion, and heartbeat, and similar complexes can be found in the brains of reptiles, hence the name.
  • The mammal brain is concerned more about safety, it keeps track of danger and keeps guiding us back to old habits, where life is safe.
  • To change our habits, we need the attention of the crucial prefrontal cortex where executive functions such as rational thinking, troubleshooting, reasoning, and more take place.

Our brains didn’t develop in a day. It has taken millions of years for the human brain to reach its current state, with different parts evolving at different times.

According to the Triune Brain model, developed by Paul MacLean (1990), the most ancient centres of our brains are grouped into the so-called paleo-reptilian complex (or the lizard brain). The lizard brain controls our vital functions, such as breathing, digestion, and heartbeat, and similar centres can be found in the brains of current reptiles, hence the name.

Aquaria on Shutterstock
Source: Aquaria on Shutterstock

The Lizard Brain and The Mammal Brain at Work
On top of the lizard brain, we find another cluster of more recent centres, responsible for keeping us safe, called the paleo-mammalian complex (or the mammal brain). The mammal brain enables us to acquire skills and habits, creating automatic routines to ensure quick reaction times. It rules the majority of our unconscious mental processes such as walking, driving a car, making coffee in the morning, reacting when somebody insults us during the day and many more.

Since safety is the mammal brain’s main goal, it constantly tracks potential dangers and prefers us to stick to our current habits because they have resulted in our survival up to this point. If threatened, the mammal brain creates emotions we don’t like, such as anxiety, fear, or anger, to steer us away from danger or to get us back to the same old routines. The mammal brain’s job is to keep us safe and save energy, and so it creates and maintains very powerful neural networks, which in turn automate our behaviour by creating habits. The mammal brain learns by repetition, creating the strongest brain networks for the actions we repeat most often. These are the crucial centres for what we call the unconscious mind, and all other mammals have similar centres too.

If we want to change our habits, we need to use another brain region, the neocortex (or the human brain), especially the very front part of it, called the prefrontal cortex (PFC). The neocortex is the largest part of the brain and is more developed than in any other animal, hence it is called the human brain. This structure is divided into many parts, each responsible for different functions. We will meet some of these centres in future posts since they are crucial for how we perceive the world and act in it. But just to give you a quick summary: the frontal lobes enable rational thinking and voluntary movements, the parietal lobes sense touch, temperature, and taste, the occipital lobes are responsible for vision, and the temporal lobes control hearing and smell.

The Prefrontal Cortex Is Important in the Quest to Change Our Habits
In terms of changing habits, the most crucial structure is at the very front of the frontal lobes, hence its name, the prefrontal cortex. The PFC is involved in a wide range of mental abilities that enable us to get things done throughout the day – the so-called executive functions of the brain: rational thinking, troubleshooting, analysing data, reasoning, learning new information, rational decision-making, creativity, and more. It is pretty much everything we call rational intelligence. The PFC is also crucial for deciding what changes we want to create in our lives and it enables us to make these changes happen. The PFC is the main area governing willpower, creating the ability to delay gratification and wait for long-term rewards, resisting temptations on the way.

The brain has a system of priorities when it comes to ‘feeding’ these different parts, which consume different amounts of energy. The lizard brain is the most efficient, and the human brain is the most energy-consuming since it is responsible for complex functions that need the activity of thousands of neurons, requiring lots of glucose and oxygen to ‘feed’ them.

Here is a metaphor of different motor vehicles to illustrate the difference in energy consumption of these centres. The lizard brain is like a small motorcycle; it is always on and it uses energy very efficiently, thus the brain always finds nutrients and oxygen for our lizard brain since if it didn’t we would die. The mammal brain is like a light car; it’s a lot more expensive fuel-wise than the lizard brain, but is still active for most of our waking hours, controlling our automatic habits. Some centres of the mammal brain are also active during sleep to process the information we encounter during the day. Last but not least, the human brain is like a plane; it uses enormous amounts of nutrients and oxygen, thus is active only when two conditions are met: First when we are carrying out tasks that require these functions and second when the brain has enough fuel left after feeding the other two brain centres.

Waiting Until the End of the Day to Address Old Habits May Be a Bad Idea
This implies that at the end of the day, when we are tired, and especially if we have been stressed, there is not enough energy for the human brain, particularly its most complex parts such as the PFC, to function optimally. For this reason, we revert to old habits, which are governed by the more energy-efficient mammal brain. Therefore, if we want to start new habits, waiting until the end of the day is a bad idea. Our willpower (also governed by the PFC) is at its lowest at that point, too. This phenomenon is called ego depletion. (Note that the concept of ego depletion has failed to replicate, and has been under some debate.)

Willpower and other important qualities of the PFC are compromised at the end of the day when the brain has little energy left and the PFC needs time off. As a result, we are more prone to give in to temptations or choose whatever path is easiest. Doing things in new ways requires us to use the more developed centres of the brain, for which a lot more energy is needed. Therefore, the first rule for creating new habits is to do new things either in the morning or straight after a break.


This post is adapted from the book "Why The F*ck Can’t I Change?"

Barros, L. F., Bolaños, J. P., Bonvento, G., Bouzier-Sore, A. K., Brown, A., Hirrlinger, J., Kasparov, S., Kirchhoff, F., Murphy, A. N., Pellerin, L., Robinson, M. B., & Weber, B. (2018). Current technical approaches to brain energy metabolism. Glia, 66(6), 1138–1159.

MacLean, P. D. (1990). The Triune Brain in Evolution: Role in Paleocerebral Functions. Plenum Press.

Toleikyte, G. (2021). “Why The F*ck Can’t I Change?: Insights from a neuroscientist to show that you can”. Thread

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