Struggling to understand things that seem simple to everyone else is surprisingly common, and it almost never means something is wrong with your intelligence. The causes range from anxiety and poor sleep to nutritional deficiencies and undiagnosed conditions like ADHD. Understanding why this happens starts with recognizing that “simple” comprehension actually demands a lot from your brain, and dozens of factors can quietly interfere with that process.
Your Working Memory May Be Overloaded
Most of what feels like “understanding” relies on working memory, the mental workspace where you hold and manipulate information in real time. When someone gives you directions, explains a concept, or asks you to follow a conversation, your working memory is doing the heavy lifting. It stores the pieces temporarily, connects them, and helps you make sense of the whole picture. When this system is stretched thin, even straightforward information can feel confusing.
Working memory has a limited capacity in everyone, but certain conditions shrink it dramatically. ADHD is one of the biggest culprits. Research published in Neuropsychology found that 75% to 81% of people with ADHD show measurable deficits in central executive working memory, the component responsible for actively organizing and manipulating information. These weren’t small differences. The impairment was classified as “very large magnitude,” meaning the gap between people with ADHD and those without was substantial. If you’ve never been evaluated for ADHD but consistently struggle to follow instructions, retain what you just read, or keep track of conversations, this is worth exploring.
Anxiety Takes Up Mental Bandwidth
Anxiety doesn’t just make you feel nervous. It actively competes for the same cognitive resources you need to understand things. Attentional Control Theory, a well-established framework in psychology, explains how this works: anxiety shifts your brain away from focused, goal-directed thinking and toward a reactive, threat-scanning mode. Instead of concentrating on what someone is saying, your mind drifts to a worried thought, a physical sensation like a tight chest, or a vague sense that something is wrong.
This effect hits comprehension harder than simpler tasks. You might be able to read words on a page just fine, but actually understanding and retaining what they mean requires more cognitive resources, exactly the kind anxiety steals. So if you notice that your ability to grasp things worsens during stressful periods, or that you can technically “hear” someone talking but their words don’t seem to land, anxiety is a likely factor. The frustration of not understanding can itself create more anxiety, feeding a cycle that makes the problem worse.
Sleep, Nutrition, and Brain Fog
Your brain’s ability to process information depends heavily on its physical state. Chronic sleep loss, even moderate amounts over weeks, degrades attention, memory consolidation, and the mental flexibility needed to make sense of new information. Interestingly, research on short-term sleep deprivation (around 24 hours) shows that basic reaction time and simple attention tasks often hold up, but more complex reasoning and comprehension suffer. This means you might feel “functional” on poor sleep while your ability to truly understand things quietly deteriorates.
Nutritional deficiencies create a similar slow erosion. Vitamin B12 deficiency, in particular, is linked to significant cognitive dysfunction including poor focus, forgetfulness, difficulty concentrating, and mental fatigue. A study published in Cureus found clear associations between low B12 levels and progressive cognitive impairment, even at levels considered “low-normal” rather than clinically deficient. People with B12 deficiency often describe feeling foggy, lethargic, and unable to think clearly, all of which make simple tasks feel inexplicably hard. B12 deficiency is especially common in vegetarians, vegans, older adults, and people taking certain medications that reduce stomach acid.
Brain fog itself has become a recognized public health concern, with causes spanning long COVID, autoimmune conditions, hormonal changes (including pregnancy and menopause), fibromyalgia, kidney dysfunction, and chronic inflammation. Long COVID is one of the most common modern triggers; brain fog is among the most frequently reported symptoms of both acute and long-term COVID infection, and it can persist for years. If your comprehension difficulties started after an illness or alongside other symptoms like fatigue and body aches, an underlying medical condition may be driving them.
Processing Speed and Learning Differences
Some people process information more slowly than others, not because they’re less capable, but because their brains need more time to decode and organize input. This is a core feature of several learning differences. Dyslexia, for example, is a neurobiological condition that affects how quickly and accurately you can decode written information. A growing body of research confirms that children and adults with dyslexia often show measurable processing speed impairments. The underlying principle is straightforward: if your brain takes longer to process each piece of incoming information, you hit a bottleneck. There’s a limit to how much you can take in during a given time window, and when information comes faster than you can process it, comprehension breaks down.
This doesn’t show up equally across all situations. You might understand things perfectly well when you have time to read at your own pace or when concepts are explained slowly, but struggle in fast-paced conversations, meetings, or lectures. That gap between your ability and the speed of incoming information is what creates the feeling of “not getting” something simple.
Auditory processing disorder is another condition worth knowing about. People with this condition have completely normal hearing, but their brains have difficulty interpreting spoken language, especially when it’s complex or delivered in noisy environments. Common experiences include trouble following phone conversations, difficulty with multi-step verbal directions, and frequently needing people to repeat themselves. Adults with auditory processing disorder often struggle most at work, where fast-paced verbal communication is the norm.
Executive Dysfunction Beyond ADHD
Executive function is an umbrella term for the brain’s management system. It covers working memory, cognitive flexibility (how easily you shift between ideas), and inhibition control (how well you steer your thoughts and stay on task). These three abilities form the foundation of understanding anything, because comprehension requires you to hold information, connect it to what you already know, and filter out distractions.
Executive dysfunction can occur with ADHD, but it also shows up with depression, traumatic brain injury, autism, chronic stress, and aging. Cleveland Clinic identifies the core signs as difficulty planning, trouble shifting between tasks, problems with reasoning through multi-step processes, and an inability to hold information in mind long enough to use it. If you find that you can understand individual pieces of information but struggle to assemble them into a coherent whole, or if you lose track of where you are in a conversation or set of instructions, executive dysfunction is likely involved.
When It’s Worth Getting Evaluated
Occasional difficulty understanding things is normal and usually tied to stress, fatigue, or distraction. The point at which it becomes worth investigating is when the pattern is persistent and affects your daily life: your work performance, your relationships, your ability to manage routine tasks. If you notice a significant decline from how you used to function, or if these difficulties have been present for as long as you can remember but you’ve always just assumed you weren’t smart enough, both of those are reasons to seek an evaluation.
A neuropsychological assessment can measure your working memory, processing speed, attention, and executive function against population norms. This isn’t a pass-fail intelligence test. It’s a detailed map of how your brain handles different types of information, and it can identify specific bottlenecks that explain your experience. Blood work can rule out B12 deficiency, thyroid problems, and other metabolic causes. If your difficulties started suddenly or have worsened noticeably over months, that pattern is especially important to bring to a clinician’s attention, as it can point toward treatable medical conditions rather than longstanding cognitive traits.
The most important thing to understand is that struggling with comprehension is almost always about how your brain processes information, not about how much you’re capable of understanding. The right support, whether that’s treating an underlying condition, learning strategies that work with your processing style, or simply getting more sleep, can make a meaningful difference.