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The message may not be new, but the importance of heating bedrooms overnight does not appear to be filtering through, at Dunedin local government level at least.

Cosy Homes project manager Jordana Whyte asked Dunedin city councillors and those in the public gallery at a committee meeting this week to put their hands up if they heated their bedrooms at night to make sure temperatures did not drop below 16degC.

In a room of about 20 people, few, if any hands got closer to the ceiling than they were before the question was asked.

But Ms Whyte said keeping bedrooms warm at night, no matter how many blankets were used or how warm electric blankets or hot water bottles kept the sleeper, could be a matter of life and death, especially for the very young and very old.

Studies have shown hospitalisation and deaths rise in winter, with cold environments one reason.

Ms Whyte said based on World Health Organisation minimum standards, bedrooms should be heated to at least 16degC, while the minimum for small children and the elderly was 18degC.

She also pointed to a West Midlands Public Health Observatory study that showed in older people, blood started to thicken when the air was at 12degC.

More than two hours at 12degC raised blood pressure and increased the risk of heart attack and stroke.

At that temperature veins contracted and the heart had to work harder to pump blood.

In New Zealand there was an excess of deaths in winter compared to summer, about 1600 more deaths a year.

''That's significant.''

She said the problem was more common in places like New Zealand and the United Kingdom.

While it was counterintuitive, that was because they were less cold than some other countries.

That meant they often did not have adequate insulation and central heating.

In much colder places, like Sweden, people ''would literally die in the cold in the winter'' without proper heating.

The problem could also be to do with attitudes towards cold.

One councillor at this week's meeting noted family members often slept outside in temperatures under 10degC, and came to no harm.

Ms Whyte responded after the meeting people were disinclined to believe the recommendations.

''So much research and science has gone into this.

''I encourage people to try it, because I think they might notice a difference in how they feel, and how often they're getting a cold or chest infection in winter.''

In terms of the cost of nighttime heating, Ms Whyte said there were things people could do to keep the cost down.

Ceiling and under-floor insulation was the first step, and good curtains made sure windows were insulated against heat loss.

Ventilating homes daily was also important, as was reducing moisture by not drying clothes inside.

Those actions would make it much more cost-effective to heat a bedroom at night. However, ''you can't just choose one of those things, you have to do all of those things''.

''They all work together so the house can perform properly.''

Ms Whyte's suggestions were backed up by Prof Philippa Howden-Chapman, professor of public health at the University of Otago, Wellington.

Prof Howden-Chapman said the advice coming from the World Health Organisation, though not yet a formal recommendation, was homes should be kept at 18degC.

It was the same advice as had been given before and was ''based on a whole lot of evidence.

''I would say it's particularly important if there's a young child in the bedroom, or someone with chronic illness or an older person, definitely think about keeping the bedroom warm.''

Prof Howden-Chapman said sometimes people said they were ''nice and warm'' in their bed, and the warmth of the room did not matter.

People often ask the question: ''If you're warm in bed, isn't that enough?''

She pointed to research done in which one group of people slept in rooms heated to between 18degC and 21degC, while another slept in rooms in which the temperature dropped to below 12degC.

Participants were allowed to use as many clothes or blankets as they wanted.

Those people who were in the cold temperatures had blood pressure ''considerably higher'' in the morning.

The study had been done with healthy students, so it affected everyone, but particularly the very young and the elderly.

david.loughrey@odt.co.nz