What a concussion is
Mayo Clinic describes concussion as a mild traumatic brain injury that affects brain function. During a concussion, the brain moves back and forth against the inside of the skull, either from a blow to the head, neck or upper body, or from sudden acceleration or deceleration, as in a car crash, a fall from a bike or a sports collision. Falls are the most common cause in adults and very young children.
The effects are often short term, and most people recover fully. Not everyone who has a blow to the head has a concussion, and Mayo Clinic notes that most concussions do not cause loss of consciousness. The NHS explains that most head injuries are not serious, but it is important to get medical help for any symptoms.
Physical symptoms
Mayo Clinic and the CDC list physical symptoms that may follow a concussion:
- headache
- nausea, upset stomach or vomiting, especially early on
- dizziness or balance problems
- tiredness, drowsiness or lack of energy
- blurred vision or other vision problems
- ringing in the ears
- sensitivity to light or noise
A witness may notice a temporary loss of consciousness, though it does not always occur, slurred speech, delayed responses, a dazed look or the person asking the same question repeatedly. See also our guide to dizziness and headache.
Thinking, mood and sleep symptoms
The CDC groups other symptoms into three areas:
- Thinking and remembering: attention or concentration problems, feeling slowed down, foggy or groggy, memory problems and trouble thinking clearly
- Social or emotional: anxiety or nervousness, irritability, feeling more emotional and sadness
- Sleep: sleeping more or less than usual and trouble falling asleep
Mayo Clinic adds confusion, feeling as if in a fog, and amnesia around the event: people may not remember what caused the injury.
Delayed symptoms
Both Mayo Clinic and the CDC stress that signs and symptoms may not show up right away and can take hours or days to appear. Mayo Clinic lists symptoms that sometimes appear days later: trouble with concentration and memory, irritability and personality changes, sensitivity to light and noise, sleep problems, feeling emotional or depressed and changes in taste and smell.
The CDC explains that symptoms differ between people, may change during recovery and are usually most severe in the first 1 to 2 days. The NHS notes that symptoms may last up to 2 weeks after a head injury. Mayo Clinic adds that symptoms can last for days, weeks or longer, and that some people have symptoms for longer than expected; when they last more than three months, they are called persistent postconcussive symptoms.
Signs in children vs adults
Concussion can be harder to recognize in young children and some children with disabilities, who may not be able to say how they feel. The CDC lists signs in children aged 0 to 4, including:
- irritability and crying more than usual, or seeking more comfort
- refusing to nurse or eat, and changes in sleeping patterns
- losing interest in play, or needing more help than usual
- vomiting right after the injury without another cause
- appearing dazed, confused, clumsy or unsteady
In children and teenagers aged 5 to 17, the CDC lists appearing dazed or confused, sudden difficulty remembering or concentrating, slower answers and reactions, clumsiness, vomiting and mood or behavior changes. Mayo Clinic advises that children and adolescents be assessed by a professional trained in pediatric concussion. The CDC page focuses on children, so its guidance should not be applied directly to adults.
Red flags: go to emergency care
The NHS advises calling emergency services if someone has hit their head and:
- has been knocked out and has not woken up, or cannot stay awake or keep their eyes open
- has a seizure
- has problems with vision or hearing, a black eye without hitting the eye, clear fluid from the ears or nose, or bleeding from or bruising behind the ears
- has new numbness or weakness, or problems walking, with balance, understanding, speaking or writing
- fell from more than 1 meter or 5 stairs, or hit their head at high speed
- has a head wound with something inside it or a dent in the head
- shows behavior changes, or is under 1 year old with a bruise, swelling or cut on the head
Mayo Clinic and the CDC add repeated vomiting, a headache that gets worse, increasing confusion, agitation, not recognizing people or places, slurred speech and one pupil larger than the other. The NHS advises seeking urgent advice for vomiting, dizziness, blood-thinning medicines or alcohol or drug use at the time of injury, and not driving yourself to hospital.
What doctors check
Mayo Clinic advises seeing a healthcare professional within 1 to 2 days after any head injury, even when emergency care is not needed. This does not mean waiting if there are danger signs: those need emergency care straight away. The NHS advises seeing a GP if symptoms last more than 2 weeks, or if you are not sure whether it is safe to drive or return to work, school or sport.
Mayo Clinic explains that a traumatic brain injury can sometimes cause bleeding in or around the brain, leading to prolonged drowsiness and confusion, which is why anyone with a head injury should be monitored in the hours afterward and seek emergency care if symptoms worsen.
The first days: rest and gradual return
For a minor head injury cared for at home, the NHS advises:
- applying a wrapped ice pack to the area for short periods in the first few days to reduce swelling
- resting and avoiding stress; there is no need to stay awake if you are tired
- taking simple painkillers such as paracetamol for headaches, as directed
- making sure an adult stays with the injured person for at least the first 24 hours
- not drinking alcohol or taking drugs, and not taking sleeping pills unless a doctor advises it
Mayo Clinic notes that, depending on symptoms, people may need to return to learning and physical activity gradually. The return to activities should be stepwise and guided by a professional, rather than a sudden return once you feel better.
Where the sources differ
The three sources agree on the main symptoms and danger signs but frame them differently. Mayo Clinic (July 2026) lists loss of consciousness longer than 30 seconds among emergency signs, while the NHS (May 2025) advises emergency care for anyone knocked out who has not woken up and urgent advice for other symptoms; Mayo Clinic also advises seeing a professional within 1 to 2 days after any head injury. The CDC page (September 2025) is written for parents and focuses on children.
On return to activity, the NHS advises no contact sports for at least 3 weeks and no driving until fully recovered, while Mayo Clinic stresses never returning to play on the same day and a gradual return supervised by a professional. Mayo Clinic also warns of the rare but potentially fatal second impact syndrome, rapid brain swelling after a second head injury before recovery from the first.
In summary
Concussion is a mild traumatic brain injury that can cause headache, dizziness, nausea, confusion, memory problems and changes in mood and sleep, sometimes appearing hours or days later. Most people do not lose consciousness and recover fully. Seek emergency care for danger signs such as being knocked out, seizures, repeated vomiting, a worsening headache, weakness, slurred speech or unequal pupils.
For a companion, it helps to note the time of the injury, how it happened, whether the person lost consciousness and for how long, the symptoms observed and any changes over the following hours, and to share these with the professional. Return to school, work, driving and sport gradually, never on the same day as the injury. MediBilbao Salud does not offer emergency or concussion care.
Frequently asked questions about concussion symptoms
These answers are general information and do not replace an examination or individual medical advice.
What are the first signs of a concussion?
Headache, confusion, dizziness, nausea or vomiting, blurred vision, a dazed look, slowed responses and memory gaps around the event.
Can concussion symptoms appear later?
Yes. Symptoms may take hours or days to appear, such as concentration problems, irritability, sensitivity to light and noise or sleep changes.
Do you have to lose consciousness to have a concussion?
No. Most concussions do not cause loss of consciousness, so a concussion cannot be ruled out because someone stayed awake.
When should you go to the emergency department?
For loss of consciousness, seizures, repeated vomiting, a worsening headache, weakness or numbness, slurred speech, confusion, unequal pupils, fluid from ears or nose or a high-energy injury.
How long do concussion symptoms last?
Often days to a couple of weeks, and they are usually worst in the first 1 to 2 days. See a doctor if they last more than 2 weeks.
When is it safe to return to sport?
Never on the same day. The NHS advises avoiding contact sports for at least 3 weeks, and Mayo Clinic advises a gradual return supervised by a professional.
Sources
- Mayo Clinic · Concussion: symptoms and causes
- NHS · Head injury and concussion
- CDC HEADS UP · Signs and symptoms of concussion
This guide is for general information and does not replace medical advice. How a problem is diagnosed and treated depends on your examination and medical history. If you think you have an emergency, contact your local emergency services.