His nursing career has dictated this pattern for decades - and he knows it's toxic.

"Your sleep patterns are absolutely crap, you run on three to four hours of sleep the first couple of nights then you're half awake and half asleep."

Ken, whose name Stuff agreed to change to protect his employment, said it gets harder every year. "I've always been told shift work knocks five years off your life... I believe it."

Graveyard shifts or starts before 7am are among the top 10 carcinogens affecting more than half of New Zealand's workforce, according to the New Zealand Carcinogens Survey.

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