Most experts recommend washing bed sheets once a week, with every two weeks considered the outer limit. Sheets may need cleaning more often after illness or periods of heavy sweating, despite any suspicion that such advice sounds excessively fussy.

Some alarming online claims deserve more skepticism. A statement that bed sheets are “filthier than a toilet bowl,” for example, may come from publicity created by a bedding company rather than a study conducted by microbiologists and reviewed by peers.

That does not mean an unwashed bed stays pleasantly clean. Sheets and pillowcases can become crowded with microorganisms and other unwanted material when they are left in place for too long.

The average person spends about one third of life in bed, whether sleeping, trying to sleep, or pursuing other activities. During those prolonged stretches beneath a duvet, sweat, bodily oils, and dead skin cells steadily collect on surrounding fabrics.

Comparing sheets with clothing offers a blunt reminder of what can accumulate. Fabric that remains in close contact with the human body can quickly pick up odors, stains, and other unpleasant traces.

Microbial life adds another layer to the problem. Bacteria and fungi can accumulate in a bed alongside all the material deposited directly by the person sleeping there.

Sharing a bed with a cat or dog brings still more additions. Hair, dander, dirt, faecal matter, and a wider variety of microbes can all join the mixture gathering in the bedding.

Hospital beds can contain particularly undesirable organisms. Staphylococcus aureus, a widespread bacterium capable of causing skin infections, and E. coli, a similarly common bacterium associated with food poisoning and poop, are among the potential residents.

Scientific evidence about the microbiome of an average bed remains surprisingly limited. However, studies suggest homes contain the same microbial communities found on human skin, making it reasonable to imagine those communities accumulating in beds at even greater concentrations.

This domestic microbiome may also be remarkably persistent. A 2014 study described a “microbiological aura” that follows people when they move from one home to another.

Public washing habits show that many people are not following the weekly recommendation. A 2022 YouGov poll in the UK found that 28% of respondents washed their bed sheets every week.

The largest group in the poll followed a less frequent schedule. Some 36% washed their sheets once every two weeks, while 9% did so every three weeks and 11% waited four weeks.

The ends of the scale were even more striking. Around 4% said they would leave their bed sheets unchanged for two months or longer, while 2% washed them more than once each week.

Whether there is a scientifically proven optimal schedule is less clear because the available evidence is sparse. Still, the 28% of people washing sheets once a week appear to be following a sensible rule of thumb consistent with expert recommendations.

Illness and heavy sweating provide reasons to clean sheets more frequently than the standard weekly schedule. The combination of prolonged bodily contact and accumulating biological material helps explain why regular washing is more than a matter of keeping fabric crisp.

Pillows also deserve attention rather than indefinite use. The advice extends beyond changing pillowcases, with pillows themselves needing replacement every few years.

A 2005 study examined samples from 10 pillows that had been used for periods ranging from 1.2 years to as long as 20 years. Researchers concluded that the older pillows carried a “substantial load” of numerous fungal species.

Those fungi included Aspergillus fumigatus, Aureobasidium pullulans, and Rhodotorula mucilaginosa. Because people spend so much time with their faces pressed against pillows, the study concluded that the fungi could affect respiratory health, particularly among people with asthma.