What is the Window of Tolerance?
The Window of Tolerance is a concept developed by psychiatrist Dr. Dan Siegel (1999) to describe the optimal zone of arousal in which a person can function most effectively. When we’re within this “window,” our body and mind are working together in a steady rhythm to meet the changing demands of daily life. We feel grounded enough to face challenges, flexible enough to listen to our body’s needs, able to respond rather than react, and safe enough to rest when needed. This balanced state is often described as being regulated.
Two key players in this process are the branches of the autonomic nervous system: the sympathetic branch, which mobilizes us for action (often referred to as “fight or flight”), and the parasympathetic branch, which supports rest and recovery (often referred to as “rest and digest"). Within the window, these systems are operating in dynamic balance by adjusting their activity in response to what’s needed in the moment. This flexibility allows us to think clearly, cope with stress and stay connected to what's happening around us.
What Happens Outside the Window?
It is completely normal for the nervous system to move in and out of this window every day. Hyperarousal and hypoarousal are two natural states the body can move into:
- Hyperarousal (fight-or-flight): a state of heightened activation, where the body prepares to respond to a challenge. This might show up as racing thoughts, muscle tension, anxiety, restlessness, irritability, or a strong urge to push through exhaustion.
- Hypoarousal (shutdown): a low energy state designed to conserve resources. This might feel like numbness, brain fog, heaviness, or a sense of disconnection.
Both of these states are protective responses. A burst of hyperarousal can help us take action quickly when needed, and a brief dip into hypoarousal can help us rest and reset.
Difficulties arise when the nervous system struggles to return to balance and becomes stuck in one of these states for longer that then body can cope with. For example, someone living with fatigue might experience hyperarousal after a stressful moment: their heart races, breathing becomes shallow, and thoughts like “I can’t cope” take over. If the system can’t rebalance, it may swing into hypoarousal, where the body shuts down to conserve energy. Over time these prolonged survival states can reinforce cycle of overwhelm and exhaustion.
How Ongoing Challenges Can Make the Window feel Narrower
For people living with health challenges such as, ME/chronic fatigue syndrome, long COVID, chronic pain, or burnout, the window of tolerance - the zone in which the nervous system can respond flexibly and recover quickly - can often feel narrower. This narrowing happens because ongoing physical, emotional, or cognitive stress keeps the nervous system in a heightened state of sensitivity.
Repeated exposure to stress or exhaustion “trains” the nervous system to stay on guard. Similarly, repeated periods of low energy or shutdown can make it harder to re-engage when energy is available. These responses are your body's way of protecting and preserving resources, helping you cope with ongoing challenges as best it can.
As a result smaller demands, such as answering messages, planning meals or coping with sensory input, that previously caused only brief activation can now push the system into prolonged states of hyperarousal or hypoarousal. Over time, this narrowing of the Window of Tolerance can create a cycle where fatigue, overwhelm and emotional strain reinforce each other.
Why Awareness Matters
Developing awareness of your own window of tolerance is the first step in supporting a regulated nervous system.
By gently noticing the signals that indicate you are moving outside your window of tolerance, such as tension, racing thoughts, or a sense of fogginess, it becomes possible to respond with care and compassion rather than automatically reacting.
For instance, noticing the early signs of hyperarousal might prompt taking a brief pause: stepping away from screens, slowing breathing, or grounding attention in the present moment. Recognising the onset of hypoarousal (feeling heavy or disconnected) might instead call for gentle stimulation, listening to music, stretching, or connecting with someone supportive.
Whilst this might sound simple in theory, it can be challenging in practice. Often, having support in becoming familiar with your own nervous system can make this process easier. This is what we do at Flourish with Fatigue: we support you in becoming more in tune with your own nervous system - developing awareness and understanding to recognise different states, why they are activated and how to respond to them. With guidance, it becomes easier to notice shifts in your system, respond with care, and gradually build strategies that make navigating symptoms and daily life feel more manageable and sustainable.
Final thoughts
When the body has been living under chronic strain, it’s understandable that the nervous system becomes more reactive. These responses are not your fault, they are the body’s attempts to protect and preserve energy. With awareness and compassionate practice, it becomes possible to notice these shifts earlier and guide the system back toward balance, creating moments of greater stability, clarity, and calm.
References:
Siegel, D. J. (1999). The Developing Mind: Toward a Neurobiology of Interpersonal Experience. New York: Guilford Press.
Khammissa, R. A. G., Meyerov, R., Ballyram, R., Feller, L., & Fourie, J. (2022). Burnout phenomenon: Neurophysiological factors, clinical features, and treatment. Frontiers in Psychology, 13, 947869.