When radiotherapy is applied to the brain, a cancer patient can experience a whole slew of uncomfortable and distressing side effects, from headaches and nausea to hearing loss and seizures.

But one side effect doctors have never seen before just cropped up in a 59-year-old Australian man undergoing treatment for eye cancer - after dreaming in black and white for his entire life, radiotherapy suddenly changed his dreams to full colour.

Led by radiation oncologist Michael McKay from the North Coast Cancer Institute and the University of Sydney in Australia, researchers have reported how the patient's dreams switched from black and white to colour for several weeks, before reverting back to normal once the treatment had stopped. 

"During the first week of radiotherapy, he developed 'rapid', vivid-coloured dreams, which persisted throughout the course of his radiotherapy," the researchers report.

"A few days after radiotherapy, the dreams reverted to black and white and became less intense over subsequent months before ceasing."

If the idea of dreaming exclusively in black and white seems weird to you - congratulations, there's a good chance you're young. 

A 2008 study by Eva Murzyn, a psychologist from the University of Dundee in Scotland, found that while subjects 25 years or younger were more likely to dream in colour, and almost never dreamed in black and white, those over 55 were five times more likely than the 25-and-under group to dream in black and white.

Overall, she reported, just 12 percent of people were found to dream entirely in black and white. But this hasn't always been the case.

Back in the 20th century, researchers found that the "vast majority" of people surveyed actually dreamt in black and white instead of in colour.

A 1942 study reported that only 29 percent of the university students they surveyed had the occasional coloured dream, and by 1956, researchers were claiming that as little as 15 percent of dreams contained colour.

But by the 1960s, everything changed - coloured dreams suddenly came to the fore.

One 1962 study wrote that "with careful interrogation close to the time of dreaming, colour was found to be present in 82.7 percent of the dreams", and six years later, another report found that coloured dreams were experienced by 69 percent of patients who were woken up from an REM sleep.

So what changed? 

While it's all just speculation at this point, researchers are quick to point out the fact that by the early 20th century, black and white films were prevalent enough to possibly have an effect on how people were dreaming. 

"It was very likely that the average college student (the typical participant in these studies) had regular contact with black and white media," Murzyn reports.

According to the hypothesis, with the rise of coloured films and televisions came the rise of coloured dreams - by the late 1960s, nearly all movies in the US were produced in colour.

"The first colour TV shows were broadcast in 1950 (to be viewed in public places) and the first consumer colour TV sets appeared in 1954, and by 1972 the majority of US households had a colour TV," Murzyn points out.

This could well explain why our 59-year-old Australian cancer patient reports only ever dreaming in black and white - by the time Australian televisions had been converted to full colour in 1975, he was already in his 20s.

But it doesn't explain why radiotherapy to the brain would suddenly change everything.

His doctors report that during his month-long treatment, his technicolour dreams featured everything from three-dimensional algebra equations, a timeline of cars he's owned, and various species of fish he'd caught.

"The neurological events were dreams rather than hallucinations, since they immediately stopped when the patient woke up," the researchers explain.

Unfortunately for all of us, it hadn't occurred to the team to record the man's electrical brain activity on electroencephalography (EEG) tests, which would have given them a better idea of where the radiation was affecting his brain.

But the idea of radiation pulses being behind the sudden colour change isn't without merit - as Alice Klein reports for New Scientist, people exposed to radiation during the Chernobyl disaster showed unusual brain activity EEG tests, and over a third reported abnormal dreams.

"[I] the radiation was affecting [EEG-detected] activity, then that could quite readily change the dream experience," Simon Cropper from the University of Melbourne, Australia, who wasn't involved in the study, told Klein.

The researchers say their best bet is that the tissue irritation and swelling caused by the radiation contributed to the man's symptoms - days after the treatment had ended, the colour was gone again.

Another explanation could be that the radiotherapy was waking the man up during various stages of sleep, causing his subconscious to remember the colour of the real world in subsequent dreams.

Without brain scans, we'll never know what happened in this particular case, and we should point out that there are many reasons why we should take this study with a grain of salt, including the sample size of one, and the self-reporting involved.

But the researchers are keen to see if other cases will crop up now that they've identified this as a potential side effect of brain-targetted radiotherapy. 

It's not a life-changing shift to suddenly see your dreams in full colour, but knowing how it happens could help us get one step closer to understanding how the brain generates consciousness.

The research has been published in Sleep Medicine.