A new study reveals the long term effects of mild traumatic brain injuries on workers.
Researchers at Auckland University of Technology say a third of people are still impacted at work four years later.
The study examined 245 workers four years after they had suffered a mild traumatic brain injury, like concussion.
Seventeen percent of them had stopped working or reduced their hours.
A further 15.5 percent reported experiencing limitations at work, such as struggling to concentrate, sticking to a routine and managing their workload.
Lead researcher, Associate Professor Alice Theadom, says "despite the clinical definition [of the injury], it's certainly not mild in terms of impact."
Timothy Giles has suffered two mild traumatic brain injuries, an assault 14 years ago and a bang on the head playing football four years ago, he says the impact is profound.
"Fourteen years later I still can't do a full week's work," says Mr Giles, "I'm a grown adult and I still have to go for afternoon naps."
He says the symptoms are not visible and they're isolating.
"There's a personality change," he says, "I'm a lot less energetic, a lot less kind, I suffer confusion, I struggle in social situations."
"It's impossible to have a relationship. I've destroyed a lot of friendships and put pressure on my family."
He's also sensitive to light and noise.
"The headaches are constant. I love music but I never play it at home anymore."
He says it's left him bankrupt and alone.
"I've been bankrupted, that's not unusual for people with brain injuries because we make bad decisions, disordered decisions."
Auckland Brain Injury Association manager, Steve Jenkins, says mild traumatic brain injuries are "underrated".
"They're technically mild in their clinical parameters but the downstream consequences can be quite pernicious," says Mr Jenkins.
He says it's a hidden disability.
"Because you can't see it there's an internal expectation that 'it's not much' and 'I should get back to work'", says Mr Jenkins, "And there's pressure from family and employers to get back to work."
Earlier findings showed 90 percent of adults returned to work within two months, but Associate Professor Theadom, says many of them still had ongoing problems.
"Between 22 percent and 48 percent of those will experience persistent symptoms, such as headaches, fatigue and difficulties processing information for at least one year following injury which can make it very difficult to function effectively at work."