This New Years' Day was a particularly odd day for my father in law, Jim. Not that he remembers much of it.

It was a day that Jim woke early. He went to shave, as usual. Then he found he'd lost his memory.

For hours he would repeatedly ask those around him if he'd already shaved, confused by stubble-free skin and a complete lack of recollection. He couldn't remember where he spent Christmas, either. Or last Thanksgiving. His recent travels through Australia had vanished. So too any memory of the current US president.

The turning point came when his daughter – my wife – stepped into the room to check on him.

"Is … is she pregnant?" Jim mumbled out loud, baffled.

Due in March, his daughter struggled to understand the question – any previous state of tired bewilderment now more than trivial.

"Are you joking?" she asked. "If so, it's not funny, Dad."

"Why didn't you tell me you were pregnant?" Jim accused with an insistence that was out of character.

All sorts of thoughts run through your mind when this is a member of your family. Is it a stroke? Dementia? You seize any medical knowledge you have at hand to reassure yourself things are going to be ok.

There was no sign of slurring or muscle weakness. Hands were strong, free of shaking. But months had spontaneously evaporated from Jim's mind, and surely that wasn't good.

As terrifying as the experience was for his family, Jim's prognosis turned out to be a positive one. We joke about it now in hindsight, all tests showing zero abnormalities. MRI, CT, bloods … all clean. Ironically he has little memory of the event itself. It's 'just one of those things'. A medical curiosity. A blip.

We understand that Jim had an episode of transient global amnesia, a benign condition as mysterious as it is uncommon. Fewer than 10 in every 100,000 people experience it each year, and we still have no idea what causes it.

I sought advice from Amee Baird, a clinical neuropsychologist from Macquarie University in Sydney who has come across the odd case in the past.

"Although there have been hundreds of reported cases of transient global amnesia in the medical literature, the cause is still unknown, and it is regarded as one of the most mysterious of all the neurological conditions," says Baird.

"Various triggers have been reported, including strenuous physical activity, emotionally arousing or stressful events, and a sudden change in body temperature."

Migraines appear to be a risk factor, as are various cardiovascular issues.

Baird doesn't consider herself an expert in the phenomenon, but has received several patients who have experienced this spontaneous, short term loss of memory. Like me, she finds such quirks of the brain fascinating.

Baird adds that sex can also be a trigger, a factor she explored while researching her upcoming book by New South Publishing, Sex on the Brain.

"In one study [sex] was the trigger in a third of cases," Baird says.

Which events – if any of them – preceded my father in law's experience, I'm suddenly reluctant to dig into. Even my scientific curiosity has its limits.

Fortunately there is no suggestion that Jim's temporary brush with amnesia is a symptom of ill health. We can expect him to attend many more Christmases in the future, and almost certainly remember them all for a while to come. As frightening as it is for those who witness the event, there are no known long term effects.

Given the man has been a prominent historian and story teller himself, clear memory plays an intrinsic part of his identity. It's given his family pause to be thankful, and mindful of the impact more severe losses of long term memory in the form of dementia many others face.

The most severe residual effect seems to be a slight difficulty in focussing, not to mention a son in law who insists on too many questions about the experience.

I ask Baird if she has any thoughts on what TGA might tell us about neurology in general.

"That we still have a lot more to learn about memory," she says.

"Also, the growing evidence that emotional and psychological factors can play a role in triggering TGA in many cases demonstrates the intimate link between emotion and memory."

If the statistics are anything to go by, this is a once in a lifetime experience for Jim. All of those memories have returned – Christmas, Thanksgiving, and my wife's pregnancy.

Oh, and yes. He knows who the president is again.

Mike McRae has been writing science news stories and developing educational resources for over a decade. He is the author of two books - Unwell: What makes a disease a disease? and Tribal Science: Brains, Beliefs, and Bad Ideas.

Opinions expressed in this article don't necessarily reflect the views of ScienceAlert editorial staff.