One night in 1999, Danny found himself in the middle of a fight in a nightclub toilet. He was out celebrating his friend’s 21st birthday party in London’s East End, and he was young and drunk and cocksure. “They can’t do nothing to me,” he assured his friend. The argument ended with him punching the group’s ringleader.

It was, he says, “the worst mistake I ever made.” Soon, five men were piling on top of him. Danny’s injuries were so serious that he remained in a coma for three weeks – the doctors feared he was brain dead – but eventually he woke up. With rehabilitation therapies, he eventually recovered his speech and memory, but no matter how much progress he made, one symptom remained – a terrible, crushing exhaustion.

Seventeen years later, it’s still there – a fatigue that clouds and confuses his mind, so that coping with everyday activities becomes a formidable test of endurance. He’ll start forgetting basic information, like his computer passwords, for instance, or heading home from work, he’ll find that he’s taken the wrong train or bus. A simple phone conversation with his bank will leave him irritable and drained.

“To be honest, it can come at any minute, at any time,” he tells me. “Working too much doesn’t help it, but if I don’t work at all it’s still there.” The only relief, he says, is to lie down in total silence, as his brain slowly recharges.

It took years for Danny to be told the medical explanation for that exhaustion – and yet it is by no means unique. While many of the physical and verbal problems are well known, “cognitive fatigue” is one of the most disabling symptoms of a range of neurological disorders – and an important barrier to recovering a more active life.

It can be the result of a stroke or other kinds of brain injury, or it can be the result of a neurodegenerative process like multiple sclerosis and Parkinson’s disease. But a lack of awareness and understanding of these problems means that many people are not receiving the support they need.