We just found out how often you're meant to wash your pyjamas. Oops.

It’s the dilemma currently dividing parenting forum mumsnet – the frequency with which you should wash your pyjamas.

“Is it unreasonable to think you should wash them every wear?” asked one user, sparking a thread of hundreds of comments.

You see, apparently pyjamas are basically underwear and so you should be washing them every day, according to mumsnet which is obviously essentially gospel.

Those who don’t? We’re filthy animals sleeping in a cocoon of our own sweat and dead skin cells.

“I wash my pjs every time I wear them . [I’m]confused I don’t wear underwear with them so would feel like I was wearing a pair of pants twice,” read one commenter.

“People sweat in bed, and many will go to bed without washing as they shower in the mornings, so that is enough reason for me to wash pjs after one night,” added another.

Listen: How often should you wash your bra? Post continues after audio.

“I put mine and the kids’ in the wash every morning, put clean ones on every night. Always have, didn’t know people wore them again the next night. Do you put them away in the drawer then out again? asked someone else.

Those who wash them daily may be clean, but many others (probably trying to deflect attention from their stained jammies) pointed out the resulting dirty habit.

“Gawd – utterly ridiculous and wasteful to wash pjs after every wear – the world (the privileged western one) has gone mad with hygiene,” wrote one user, while another added, “Some MN-ers washing habits completely leave me shocked. Won’t someone think of the environment?”

Others were honest about their multiple-day pyjama habit.

"I don't have the time, energy or desire to launder five pairs of pjs every night. That would be...35 pairs of pjs a week!!! We don't own that many pairs!," wrote one.

"I never go to bed without a shower, and wear pants with pj bottoms, so I wear mine for a couple of nights. You would change more often if you are unwell or very hot at night, wouldn't you?" wrote someone else.

In the comfort of others who go a day or five without changing their pyjamas, and you'll likely find plenty of people who go a week or longer wearing the same comfy top and bottoms.

"I have like three sets on the go at every one time and would wash them all every fortnight or so, so i guess when you average it it's ok," said Sarah* when pressed.

"My pyjamas are my husband's t shirts so they mostly get washed when he gets mad and needs them back so like... once a week," added Tara*.

The good news is that research shows the every-dayers are in the minority.

A 2015 study by mattress company Ergoflex surveyed 2,500 18-30 year olds about the average length of time people wear their pyjamas, and turns out women are the worst offenders.

Men wore the same pair for an average of 13 nights compared to women who slept in the same nightwear for an average of 17 nights.

Feel better yet?

However looking at the reasons why is even more interesting. Fifty four per cent of women said they alternated between multiple pairs so lost track of how long they'd been out for while 73 per cent of men said they were forced to because they didn't do the washing.

So what do the experts say? Professor Sally Bloomfield, chair of the International Scientific Forum on Home Hygiene says they can carry bacteria.

"Pyjamas are against your skin. You shed skin cells at a vast rate all the time. They are full of microorganisms. We all have skin and gut organisms that are usually not harmful on our skin and in our gut," she says.

"But if they get into the wrong place they can cause problems. Quite a lot of us carry staphylococcus bacteria, which can cause infections if they get into cuts and bruises. We all carry E Coli bacteria in our bowel. Again, most strains are not harmful. But if they get into the urinary tract they can cause infection, cystitis."

If you're in a family with small children or just other people you don't want to make sick, then regular washing (once a week) is recommended. If you live alone, you can leave it longer - if you can deal with the smell.

"The dirtier your pyjamas get, the more likely it is that things will be transferred from your bowel to your urinary tract, for example. You can’t say there’s no risk, but the risk is relatively low," she told The Guardian</em>.

A wash at 60C or 40C using a product containing active oxygen bleach will be enough to destroy any bacteria.

But be warned - if you've worn the same pyjamas for two weeks, one wash isn't a magic fix.

"Washing should get rid of most microbes, but not all if you have worn them for two weeks. The clothes won’t be hygienically cleaned because the microbes will have built up, so they will be transferred to underwear and other clothing that comes into contact with the skin," Prof Bloomfield says.

Bottom line? Once a week is fine, unless they're getting soiled.

How often do you wash your pyjamas?

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