This article was written by Anna Salleh in ABC News:

Getting five hours sleep or less a night could increase your risk of getting two or more chronic diseases as you get older, research has found.

But some experts say we should be more worried about the impact of things like smoking, high blood pressure and inactivity.

The study, published today in the journal PLOS Medicine, followed 7,000 men and women, employed by the British civil service, over 30 years, tracking the amount of sleep they had and the chronic diseases they developed.

Their sleep was measured six times between 1985 and 2016 and correlated with the chronic illnesses they developed at three particular ages.

"Short sleep duration at age 50, 60 or 70 was associated with a 30 to 40 per cent increase of multimorbidity [developing more than one chronic illness]," said lead author Séverine Sabia, an epidemiologist at the Université Paris Cité, Inserm and the University College London.

People in the study were regarded as having a "short sleep duration" if they slept five hours or less, and their risk of disease was compared to those who had the recommended seven hours.

While some evidence has linked sleep duration to individual chronic illnesses - which include heart disease, cancer, arthritis, depression and diabetes - Dr Sabia said this was the first prospective study to look at the link with multiple chronic diseases.

"In real life when people get older, they have several diseases," she said, adding that over half of older adults are estimated to have at least two chronic diseases in high-income countries.

Unsurprisingly, shorter sleep was also linked to a greater risk of death, mainly because of the increased risk of chronic disease.

The researchers also found disturbed sleep was linked to multimorbidity.

And Dr Sabia and colleagues also looked at the impact of sleeping longer than recommended.

While they found sleeping longer than nine hours appeared to be linked to more chronic disease, this trend was puzzlingly only observed in the 60- and 70-year-old group.

When they looked closer, they found the people who were sleeping longer were already unwell, and their disease could well have been the cause of their extended sleeping, rather than the other way around.

"It could be due to medication, or fatigue related to the disease," Dr Sabia said.