A concussion is a disruption in how your brain functions, triggered by a bump, blow, or jolt to the head. It doesn’t involve bleeding or a visible wound on a brain scan. Instead, the damage happens at a cellular level: the impact stretches and strains nerve fibers, setting off a chain of chemical and metabolic changes that temporarily impair normal brain activity. Understanding what’s actually going on inside your skull, and what to expect in the hours, days, and weeks that follow, can make the experience less frightening and help you recover faster.
What Happens Inside Your Brain
When a force hits your head (or your body in a way that whips your head), your brain shifts inside your skull. That movement stretches nerve fibers called axons, and the strain triggers an immediate chemical reaction. Damaged neurons release a flood of glutamate, a signaling chemical that overstimulates surrounding brain cells. This causes potassium to rush out of cells while sodium and calcium rush in, throwing off the delicate electrical balance your brain depends on to function.
Your brain then enters a kind of energy crisis. Restoring that ionic balance requires enormous amounts of fuel, so brain cells start burning through glucose at an accelerated rate. At the same time, the influx of calcium causes blood vessels to constrict, reducing blood flow to the brain by as much as 50% in some areas. So your brain is demanding more energy while simultaneously receiving less of it. This mismatch between supply and demand is the core of why a concussion produces symptoms: the brain is working harder than normal just to maintain basic operations, and it doesn’t have the resources to keep up.
This metabolic disruption doesn’t resolve instantly. It can take days to weeks for cellular chemistry to fully normalize, which is why symptoms can linger even though the original impact lasted only a fraction of a second.
Symptoms You Might Notice
Concussion symptoms fall into four broad categories, and most people experience some combination from each. Not all symptoms appear immediately. Some show up within minutes, while others emerge hours or even days later.
Physical symptoms are typically the most obvious: headaches, dizziness, balance problems, nausea or vomiting (usually early on), sensitivity to light or noise, blurred vision, and a pervasive fatigue that rest doesn’t seem to fix.
Thinking and memory symptoms include difficulty concentrating, feeling mentally foggy or slowed down, trouble thinking clearly, and problems with short-term memory. You might find yourself rereading the same sentence or forgetting what you were about to say mid-conversation.
Emotional symptoms catch many people off guard. Irritability, anxiety, sadness, and feeling more emotional than usual are all common. These aren’t signs of a separate problem. They’re direct consequences of the metabolic disruption happening in your brain.
Sleep changes round out the picture. Some people sleep far more than usual, others less. Difficulty falling asleep is common, and even when you do sleep enough hours, you may not feel rested.
Danger Signs That Need Emergency Care
Most concussions resolve on their own, but certain symptoms after a head injury signal something more serious, like bleeding inside the skull. Call 911 or go to an emergency department if you notice any of the following:
- Seizures or convulsions
- Inability to recognize familiar people or places
- Repeated vomiting
- Increasing confusion, agitation, or unusual behavior
- Loss of consciousness, extreme drowsiness, or inability to stay awake
- Slurred speech, weakness, numbness, or loss of coordination
- A headache that keeps getting worse and won’t go away
- One pupil noticeably larger than the other, or double vision
In infants and toddlers, additional red flags include inconsolable crying and refusal to nurse or eat.
How Concussions Are Diagnosed
There is no single blood test or brain scan that confirms a concussion. Diagnosis is clinical, meaning a healthcare provider evaluates your symptoms, balance, memory, reaction time, and eye movements. In sports settings, the standard tool is the Sport Concussion Assessment Tool (SCAT6), a structured evaluation approved through the 2022 Amsterdam Consensus Conference. A version for children ages 8 to 12, the Child SCAT6, adjusts the tests for younger patients. For symptoms that persist beyond the first 72 hours, a more comprehensive office-based evaluation called the SCOAT6 assesses multiple functional areas to guide ongoing management.
These assessments typically include balance tests, memory checks (like recalling a list of words), concentration tasks, and screening of how well your eyes track moving objects. The goal is to establish the severity of impairment and create a baseline for measuring your recovery over time.
Recovery: What to Do and How Long It Takes
The old advice to lie in a dark room and do nothing for days has been firmly overturned. The current international consensus is clear: strict rest until all symptoms disappear is not beneficial. “Cocooning,” as clinicians call it, can actually slow recovery.
Here’s what the evidence supports instead. For the first 24 to 48 hours, relative rest is appropriate. That means normal daily activities, reduced screen time, and avoiding anything physically or mentally intense. But even during this initial window, light activity like walking is fine and even encouraged.
After those first two days, you can begin returning to light physical activity, such as walking or stationary cycling, as long as it doesn’t significantly worsen your symptoms. The guideline is practical: if your symptoms increase by more than a mild amount and settle back down within an hour, you can continue. If the spike is more than mild or lasts longer, stop and try again once symptoms return to their previous level. Structured aerobic exercise started within 2 to 10 days of injury has been shown to reduce the risk of symptoms persisting beyond one month.
Most adults recover within two to four weeks. Children typically follow a similar timeline, with most feeling better within two to four weeks as well. If symptoms last beyond that window, a referral to a specialist experienced in brain injuries is the next step. A small percentage of people develop persistent symptoms that require targeted rehabilitation, such as vestibular therapy for ongoing dizziness or graded exercise programs for prolonged fatigue.
Why a Second Impact Before Recovery Is Dangerous
Sustaining another concussion before the first one has fully healed is the single most dangerous scenario. Second impact syndrome occurs when a second blow arrives while the brain is still metabolically vulnerable. The brain loses its ability to regulate blood flow, leading to rapid, severe swelling and a spike in pressure inside the skull. This condition is often fatal. Even impacts that would individually be considered mild can become catastrophic in combination.
This is why return-to-sport protocols exist and why no one should go back to contact activities while still symptomatic. The risk is real but entirely preventable: complete each stage of recovery before exposing yourself to the possibility of another hit.
Repeated Concussions and Long-Term Brain Health
A common concern after a concussion is whether it raises the risk of chronic traumatic encephalopathy, or CTE, the degenerative brain disease found in some former athletes. The current scientific evidence is reassuring for most people. CTE is associated with long-term exposure to repeated head impacts over many years, not with getting one or a few concussions. There is no strong evidence that a single concussion, or even occasional head impacts, leads to CTE.
That said, each concussion does appear to make the brain more vulnerable to the next one, and recovery from successive concussions tends to take longer. Protecting your brain from repeated injury, especially while still recovering, remains the most important thing you can do for your long-term neurological health.