What Causes Overthinking (and How to Interrupt It)
Most people know what overthinking feels like — the same thought turning over and over, the conversation you rehearsed twenty different ways, the decision you revisited until the original question became unrecognisable. What fewer people know is what is actually happening neurologically when this occurs, and why the usual advice to "just stop thinking about it" does nothing useful.
Understanding the mechanisms behind overthinking does not guarantee you can switch it off. But it does give you better tools than willpower alone, and it points you towards approaches that have actual evidence behind them.
What's Actually Happening When You Can't Stop Thinking
The brain has a network of regions that activate when it is not engaged in an externally focused task. This is called the default mode network (DMN), and it encompasses the medial prefrontal cortex, the posterior cingulate cortex, and the angular gyrus, among others. When you are not concentrating on something in the external world — when you are waiting, resting, or zoning out — the DMN becomes active. It is associated with self-referential thinking, mental simulation, and imagining the perspectives of others.
The DMN is not pathological. It is how the brain makes sense of experience, consolidates memory, and plans for the future. The problem arises when this system becomes overactive and defaults to a particular mode of self-referential processing known as perseverative cognition. This term, used extensively in the research of Watkins and Nolen-Hoeksema, describes thinking that is repetitive, passive, and focused on problems without moving towards resolution. It is the difference between genuinely working through a difficult situation and simply recycling it. Perseverative cognition maintains the body in a low-grade stress response — cortisol remains elevated, the autonomic nervous system stays in mild threat-detection mode — which paradoxically makes it harder to think clearly or reach decisions.
Intolerance of uncertainty plays a significant role. Research consistently shows that people who struggle with not knowing — who find ambiguity more distressing than a definitive bad outcome — are more prone to repetitive negative thinking. The brain, in attempting to resolve uncertainty by running every possible scenario, actually increases distress rather than reducing it. The simulation loop generates new questions faster than it resolves old ones.
The Common Causes of Overthinking
Anxiety is the most common driver of overthinking, and the relationship is bidirectional. Anxiety creates the intolerance of uncertainty that fuels rumination, while rumination amplifies anxiety by generating a continuous stream of threat-relevant scenarios. The nervous system responds to imagined threats in some of the same ways it responds to real ones, which is why a week of overthinking about a work situation can leave someone physically exhausted even if nothing has actually happened.
Perfectionism creates a specific flavour of overthinking centred on performance and evaluation. For perfectionists, the cognitive loop tends to focus on decisions already made — replaying past actions, identifying errors, pre-empting criticism. This is sometimes described as post-event processing, and it is particularly common after social interactions, presentations, or any situation where the person felt their performance was being assessed. Perfectionism-driven overthinking often masquerades as conscientiousness. In moderate doses it might be. At high doses it is a significant source of psychological distress.
ADHD, low mood, and a history of adverse experiences each contribute to overthinking through different mechanisms. In ADHD, working memory difficulties mean thoughts are not cleanly processed and filed — they recirculate because the brain lacks the architecture to park them reliably. In depression, the thinking style tends towards brooding, a passive and ruminative focus on why things are bad, rather than problem-solving. In people with trauma histories, the nervous system has often been trained to scan for threat, meaning the brain defaults to hypervigilant processing of potential risks — a biological adaptation that becomes a cognitive burden in environments that are not actually dangerous.
How Overthinking Becomes a Habit
Rumination becomes self-reinforcing through a mechanism that is frustratingly simple: it feels like doing something. When you are running scenarios and replaying events, there is a subjective sense of engagement, of working on the problem. The brain's reward systems are partially satisfied by the sense of effort, even when that effort produces nothing useful. Over time, turning to rumination becomes an automatic response to uncertainty or distress — a learned habit rather than a deliberate choice.
The reinforcement is also social and cultural. In environments that reward thoroughness, analysis, and preparation, the habit of extended thinking about problems is often praised. The person who anticipated every risk, who prepared for every objection, who replayed every interaction to extract learning — these behaviours look like diligence from the outside. The cost they carry internally is invisible. By the time the pattern is recognised as a problem rather than a strength, it is often deeply embedded in how the person habitually responds to difficulty.
Interrupting the Loop: What the Evidence Says
Cognitive defusion, a technique developed within Acceptance and Commitment Therapy, involves creating psychological distance from thoughts rather than trying to suppress or argue with them. Instead of treating a recurring thought as a fact to be resolved, you observe it as an event — "I notice I'm having the thought that this will go wrong" rather than "this will go wrong." Research supports defusion as an effective way to reduce the behavioural influence of repetitive thoughts without requiring you to believe they are false or force them to stop. The thought can still occur; it simply carries less weight.
Physical pattern interrupts work by engaging the body in a way that pulls activity away from the default mode network and towards sensorimotor processing. Vigorous exercise is the most studied, but even brief physical movement — particularly rhythmic bilateral activity like walking — has been shown to reduce ruminative thinking. The mechanism is partly attentional (you are focused on something external) and partly neurochemical (physical activity reduces cortisol and increases dopamine and serotonin availability). This is why people reliably report that going for a walk "cleared their head" — it is not a figure of speech.
Externalising thoughts through writing is one of the most consistently supported techniques in the research literature. Expressive writing studies, pioneered by James Pennebaker, have shown that translating internal experience into language — particularly around emotionally significant events — reduces ruminative thinking and improves both psychological and physical health outcomes. For day-to-day overthinking, the practical application is structured capture: getting thoughts out of the head and into a reliable external system, which reduces the brain's compulsion to keep recycling them. This is one reason structured planning tools — like those in the OCCO range — have a material effect on mental load beyond simple organisation. Time-boxing worry is a related technique: designating a specific, bounded period each day for deliberate attention to concerns, with the agreement (practiced rather than forced) that rumination outside this window is redirected. It sounds simple and is harder to implement than it sounds, but the evidence base is reasonable.
When Overthinking Is Telling You Something Useful
Not all repetitive thinking is noise. Distinguishing between rumination and genuine problem-solving is genuinely difficult, because they can look identical from the inside. The key difference is movement: problem-solving involves generating new information, testing options, and arriving at conclusions, even provisional ones. Rumination recycles the same content without producing new outputs. If a thought is returning repeatedly and you are not closer to understanding or resolution each time it visits, it is likely rumination.
That said, some overthinking carries signal worth attending to. Persistent discomfort about a situation — a relationship, a working environment, a decision — may be the brain's way of registering that something genuinely needs to change. The problem is not the content of the thought but the processing style. If you notice a concern keeps returning despite genuine reflection, the question worth asking is not "how do I stop thinking about this" but "what would it take to actually resolve or accept this?" Sometimes the answer is action. Sometimes it is a deliberate decision to accept uncertainty and redirect attention. Either requires a clearer mind than rumination allows — which is the argument for interrupting the loop first and returning to the question with more cognitive resource available.
