Eight really is a magic number it seems, when it comes to sleep.

There’s the optimal eight hours of shut-eye that we should all be getting (although some claim to function perfectly well on less, of course). And eight is also the figure we should keep in mind when it comes to mattresses: because we should be replacing them every eight years.

“The longer you keep the same mattress, the less comfortable it becomes,” says Lisa Bond, marketing director at Dreams. “We recommend you replace your mattress every eight years, before it reaches its snooze-by date.”

When should you change your mattress?

It would be easy to dismiss this as merely a marketing ploy – but it’s advice that’s often backed up by health experts. An old, worn-out mattress isn’t going to be very supportive, after all, and this could contribute to things like back and neck pain and trapped nerves.

There’s also the issue of comfort, which should never be underestimated. According to The Sleep Foundation, 92% of us say a comfy mattress is important for getting a good night’s sleep. And as we’re increasingly aware, bad sleep can be damaging to both our short and long-term physical and mental health.

[Related story: Change your duvet in seconds with the burrito technique – we show you how]

Hygiene horrors

Mattresses don’t just become ache-inducing, flattened versions of their former super-sprung selves as time goes by – they can also become ‘hot beds of bacteria’.

Microtech Services Ltd recently carried out tests on samples of mattresses that had been used for eight years, looking at the bacteria, yeasts and moulds in them.

“Due to the amount of human contact with the average mattress, it’s inevitable that microbes and unwanted guests will develop over time,” notes leading environmental hygiene expert, Dr Lisa Ackerley. “People tend to focus on cleaning the things they can see – such as pillows and sheets – but the mattress itself can be a hotbed of potential illness.”

“Most people would be rather surprised by the things you can find in an old mattress. Mould spores and bacteria build up over the years, and although invisible, you could be breathing in these harmful spores at night.”

This can be especially problematic for people with allergies, breathing and skin conditions and who are prone to rhinitis (runny nose).

How often should you change your pillows and duvets?

If you’ve been using the same pillows for years, you’re definitely not alone: last year, Ergoflex surveyed more than 2,000 people and found that 82% had no idea how often we’re meant to be replacing pillows (when they’re covered in stains and flat as a pancake, no?).

Experts at the Sleep Council recommend pillows are replaced every two to three years, while others – like Dr Robert Oexman of Sleep To Live Institute – suggests getting new ones every six months (yep, that does sound rather costly!), because dirt, skin cells and dust mites build up in them over time, which could worsen things like allergies and asthma.

Pillows that have lost their shape and plumpness could contribute to neck pain – not to mention poor sleep - too.

Duvets, thankfully, have a slightly longer life, with experts advising we replace them every five years or so.