Experts have warned that the intense summer heat causes excessive sweating during sleep, which may make the bed a fertile environment for the growth of bacteria and fungi.
Experts have revealed how sweating during the night can lead to bacteria growing on pillowcases, making them "dirtier than a toilet" if not washed regularly.
"Your pillowcases could be considered dirtier than a toilet bowl," Dr Gareth Nye, programme leader for medical sciences, told Wales Online. "The good news is that this bacteria is very likely to have been on you to begin with and is therefore harmless."
The medical expert encourages washing pillowcases and sheets once a week to avoid the accumulation of germs and the growth of bacteria on them.
An unwashed pillowcase can harbor 3 million bacteria after just one week, which is about 17,000 times more than the average toilet seat.
Real Simple also reported that after one week without washing, pillowcases had an average of 3 million bacteria per square inch (2.5 cm).
Most people can lose between 500 and 700 milliliters of sweat on an average night, and bed sheets or sleepwear absorb at least 200 milliliters of that sweat, according to Nye's study.
“The body replaces skin cells, which causes skin cells to shed. As people lose an average of 500 million skin cells a day (most of which come out at night), the body also produces other secretions and then you have additional fluid loss through saliva,” he added.
If ignored, other problems can arise, such as fungi or dust mites, which multiply on dead skin cells and can leave behind residue that causes allergies and asthma.”
"One study found that a typical pillow contains up to 16 different types of fungi and millions of fungal spores," he continued.
In a previous study, experts warned that making your bed immediately after waking up can increase the growth of bacteria, because moist air is trapped under the covers, which helps harmful bacteria to multiply quickly. Experts admit that people should allow their beds to breathe during the summer and change the covers frequently.
Nye suggests avoiding polyester bedding and using cotton or linen sheets.