How often should you actually wash your sheets?

Once a week for the fitted sheet and pillowcases. Every two weeks if you shower before bed and sleep alone. Every three to four weeks for the duvet cover, which touches you less.

That is the answer. The reasoning is worth two minutes.

What is actually in there

A sleeping adult loses roughly half a litre of moisture and around a gram of skin cells a night. Most of the moisture goes into the air; a meaningful part goes into the sheet, along with sebum from skin and hair, and whatever was on your skin from the day.

Dust mites eat the skin cells and need the humidity. They are harmless in themselves and a genuine problem for people with allergies or asthma.

None of this is dramatic after one night. After three weeks it is a measurably different sleeping surface, and it is what makes white bedding go dull grey — that greying is body oil, not dirt.

Wash more often if

• You sleep hot or sweat at night.
• You have eczema, asthma or dust allergies. Weekly at 60 °C is the standard advice for a reason.
• A pet sleeps on the bed.
• You go to bed without showering after a physical day.
• You use face oils or night creams — pillowcases twice a week, or the product ends up back on your face.

You can stretch it if

• You shower at night and wear pyjamas — pyjamas take a surprising share of what would otherwise reach the sheet.
• The bedroom is cool and well aired.
• You are away half the week.

Temperature

40 °C handles ordinary washing and is kinder to the fibre and the dye. 60 °C kills dust mites, which matters for allergies and after illness, but it shortens the life of the cloth and fades colour faster.

A reasonable compromise: 40 °C weekly, 60 °C once a month for whites.

The practical trick

Own two sets and alternate. Strip the bed, put the second set straight on, wash the first when convenient. It removes the "I cannot face washing the bed today" problem entirely, and two sets used in rotation last considerably longer than one set washed twice as often.

Airing is not washing

Pulling the duvet back for an hour in the morning lets the moisture out and genuinely slows the whole process. It does not replace washing. It does buy you a few days, and it costs nothing.