A concussion can cause a wide range of physical, cognitive, emotional, and sleep-related problems, some appearing within minutes and others not showing up for hours or days. Most people recover within a few weeks, but up to 30% of those with a mild traumatic brain injury develop symptoms that persist for months. Understanding the full scope of what a concussion can trigger helps you recognize problems early and recover more effectively.
What Happens Inside Your Brain
A concussion starts with a mechanical force to the head, neck, or body that stretches and strains nerve fibers in the brain. This triggers a chemical chain reaction: brain cells release a flood of signaling molecules that throw off the normal balance of charged particles flowing in and out of cells. Potassium rushes out, while sodium and calcium rush in. Your brain needs a huge burst of energy to restore order, but blood flow to the brain can drop by as much as 50% after a concussion, starving cells of the oxygen and fuel they need.
To compensate, the brain burns through its glucose reserves at an abnormally high rate. When that supply runs out, the brain shifts into an energy-depleted state. This mismatch between high energy demand and low energy supply is the root cause of most concussion symptoms. It’s why even simple mental tasks like reading or scrolling a phone can feel exhausting in the days after a head injury.
Physical Symptoms
Headache is the most common physical symptom, often starting within minutes of the injury. Beyond that, concussions frequently cause dizziness, balance problems, nausea or vomiting (especially early on), fatigue, and sensitivity to light or noise. Vision problems are particularly common. Blurred vision, light sensitivity, and double vision occur in up to 40% of children and adolescents immediately after a concussion.
Some of these visual problems have specific causes. Your brain’s ability to turn both eyes inward to focus on something close, like a book or phone screen, can weaken after a concussion. This is one of the most common visual disruptions, and it explains why reading becomes difficult: words may blur or double, you lose your place on the page, and your eyes tire quickly. Up to 50% of concussion patients in specialty clinics also develop trouble with the focusing mechanism of the eye itself, making near tasks blurry and headache-inducing. The neural pathways that control eye movement are spread widely throughout the brain, making them especially vulnerable to the diffuse stretching that occurs during a concussion.
Cognitive and Emotional Effects
The thinking-related symptoms of a concussion can be just as disruptive as the physical ones. You may have trouble concentrating, feel mentally foggy or slowed down, or struggle with short-term memory. Tasks that were once automatic, like following a conversation or solving a simple math problem, can suddenly require intense effort.
Emotional changes catch many people off guard. Irritability, anxiety, sadness, and a general sense of being “more emotional” are all well-documented effects of concussion. These aren’t signs of weakness or a separate mental health issue. They’re a direct result of the energy crisis and chemical disruption happening in the brain. Sleep also takes a hit: some people sleep far more than usual, while others develop insomnia or have trouble falling asleep. Disrupted sleep, in turn, slows cognitive and emotional recovery.
Symptoms That Need Emergency Attention
Most concussion symptoms are uncomfortable but not dangerous. A smaller set of warning signs, however, can indicate bleeding or swelling in the brain and require immediate emergency care. Call 911 or go to an emergency room if you notice any of the following after a head injury:
- Seizures or convulsions
- One pupil noticeably larger than the other
- A headache that keeps getting worse and won’t go away
- Repeated vomiting
- Increasing confusion, restlessness, or agitation
- Slurred speech, weakness, numbness, or loss of coordination
- Inability to recognize familiar people or places
- Increasing drowsiness or inability to stay awake
In infants and toddlers, inconsolable crying and refusal to eat or nurse are additional red flags.
Persistent Symptoms After Concussion
Most children with a concussion feel better within two to four weeks. Adults generally follow a similar timeline, though individual variation is significant. Up to 30% of people who sustain a mild traumatic brain injury, however, develop symptoms that linger well beyond the expected recovery window. This is sometimes called post-concussion syndrome, and it can involve ongoing headaches, difficulty concentrating, dizziness, fatigue, mood changes, or any combination of the symptoms described above.
What separates those who recover quickly from those who don’t is still an active area of study. Factors like the severity of the initial injury, a history of previous concussions, pre-existing anxiety or depression, and how early you resume light activity all appear to play a role.
The Link to Repeated Head Impacts
Chronic traumatic encephalopathy (CTE) is a degenerative brain disease caused by a buildup of abnormal proteins that damage brain tissue and kill brain cells. Research links CTE to long-term, repeated exposure to head impacts, the kind experienced over years of contact sports, military combat, or intimate partner violence. There is no strong evidence that a single concussion or occasional head impacts lead to CTE.
CTE can only be diagnosed after death, and scientists still don’t know why some people with a long history of head impacts develop it while others don’t. Genetics, sex, medical history, and lifestyle factors all likely play a role. Prior studies suggest CTE is rare, especially among people without a documented history of sustained, repeated head impacts.
Second Impact Syndrome
One of the most dangerous things a concussion can cause is vulnerability to a second injury before the first has healed. Second impact syndrome occurs when someone sustains another concussion while still recovering, triggering rapid, often fatal brain swelling. The brain loses its ability to regulate its own blood flow, leading to vascular congestion and a spike in pressure inside the skull that can be impossible to control. This is the primary reason strict return-to-activity protocols exist, and why no one should return to contact sports while still symptomatic.
Recovery and Returning to Activity
The old advice to sit in a dark room until all symptoms disappear has been replaced by a more active approach. International consensus guidelines now recommend relative rest, not strict rest, for the first 24 to 48 hours. That means you can do everyday activities like walking and light household tasks, but should reduce screen time and avoid anything physically or mentally intense.
After those first one to two days, light physical activity like walking or stationary cycling is encouraged as long as it doesn’t worsen symptoms more than mildly. A mild increase means no more than a slight bump in symptoms that resolves within an hour. If symptoms spike beyond that, stop and try again once they settle.
For athletes, the standard return-to-play protocol has six stages, each requiring a minimum of 24 hours before advancing. It begins with a return to normal daily activities like school or work, then progresses through light aerobic exercise, moderate activity with head movement, heavy non-contact training, full-contact practice, and finally competition. If symptoms return at any stage, you drop back to the previous step and rest before trying again. A healthcare provider should give the green light before you begin the progression and before you return to full competition.