Millions of people are sleeping in beds teeming with the droppings of cockroaches and dust mites.
A nationwide survey of homes in the United States, the most thorough study of its kind, has revealed a grim picture of the host of organisms that share our sleeping hours with us.
The droppings of bed bugs have been linked to human allergies, in particular, asthma.
Experts from the National Institute of Environmental Health Sciences investigated a representative sample of 830 homes in 75 areas across the US.
They used specially adapted vacuum cleaners to collect samples from pillows, bedding and mattresses.
The experts calculated that more than 45 per cent of households had levels of dust mites sufficient to cause allergies.
Twenty-three per cent of households had amounts of mites high enough to trigger asthma attacks.
In addition, cockroach droppings were found in 6 per cent of homes.
According to Patrick Vojta, who helped lead the study, the problem is not just confined to homes located in inner-city areas.
Those in suburban and rural areas are also at risk.