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It was another regrettably familiar wake-up for Sarah Hepola. Much of her memory from the previous night was blank. She remembers talking to people at a party, but then after that a shadow drops over her memories.

How did she get there, where was the stamp on her hand from? Who bought the pizza? Who was the man beside her?

“I was like, well that’s weird, I don’t know what happened… I just kind of laughed it off, it just seemed normal to me,” she recalls.

This sort of memory loss happened time and again to Hepola – and from a very early age. It often felt like “a trap door had opened underneath me… I would wake up the next day and I would be in a different place,” she says.

She was experiencing alcohol-fuelled blackouts – a colloquial term with potentially serious consequences. As the word suggests, in this state all memories of the night turn dark after a point. Some drinkers experience less severe, fragmentary blackouts where only pieces of memory are lost.

Hepola’s regular blackouts didn’t ring alarm bells for her at the time. It was only looking back that she realised she had a “messed up” relationship with alcohol, experiences she has written about in a book.

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If this type of amnesia after drinking alcohol sounds familiar, that’s because blackouts are surprisingly common: one analysis suggests that over half of university-aged drinkers have experienced some level of blackout when asked about their drinking habits, while a survey of more than 2,000 adolescents recently out of secondary school found that 20% had experienced a blackout in the previous six months.