You probably know you’re supposed to remove all traces of makeup before crashing out for the night, but how many of you are guilty of letting that routine slide once in a while?

One survey found as many as one-third of women slept with their slap on at least twice a week – after all, it’s easy to abandon your dedicated skincare regime after one too many glasses of wine. You may think twice next time you think about hitting the sack with a full face of makeup, however, once you’ve seen the damage it can do to your skin.

One brave woman has endured a month of fully-made-up sleep in an effort to pinpoint the effect skipping your skincare routine before bedtime can have on your skin. Anna Pursglove, a British journalist, took up the challenge with the help of dermatologist and medical director of the European Dermatology London clinic, Dr Stefanie Williams, who analysed the impact of 30 days of slovenliness.

During the challenge, Pursglove gave her skin a cursory wash in the shower each morning, without the use of her usual cleansers, and reapplied her makeup over the remnants of yesterday’s each day.

‘By night three I had developed a series of tiny white cysts around my eyelashes and my skin was so dry and taut, it felt like a mask,’ recalls Pursglove. ‘By the weekend, a few friends had commented that I was looking tired.’

Ten days in and a mascara-laden eyelash resulted in a swollen, painful left eye and 48 hours without eye-makeup. By the end of the experiment, Pursglove feared she’d done real damage to her skin.

‘I had visibly blocked and enlarged pores all over my nose, dry skin across my lips and cheeks and red eyelids. The white cysts had given way to some nasty-looking eyelash dandruff,’ she says.

Having had her skin analysed at the start of the experiment, Pursglove returned to Dr Williams for the results. As a 40-year old woman, Pursglove had exhibited flaws typical to a woman her age: pigmentation, broken veins, enlarged pores and wrinkles. 30 days without removing her makeup each night later, however, and the texture of her skin was 10 per cent worse on her forehead and right hand side, and 20 per cent worse on her left side, which she slept on each night. According to Williams, this was mainly due to Pursglove skipping her twice-daily moisturising routine, resulting in a five per cent drop in moisture levels.

‘Not only is the top layer of skin dry,’ says Dr Williams, ‘but a layer of make-up will also prevent normal skin shedding, slowing down the usual renewal process, resulting in uneven and dull textured skin.’

The wrinkles Pursglove already had on her forehead hadn’t worsened significantly, but had become deeper on the right and left sides of her face – the areas already identified as the driest. ‘When skin is dry, it’s less elastic so wrinkles are more prominent,’ Dr Williams explains.

According to Dr Williams, the dryness would have been compounded by environmental pollutants sticking to the residual make-up, causing oxidative stress, where skin is attacked by harmful free radicals.

‘These molecule-sized compounds cause damage to various cellular structures in the skin and can actually decrease production of collagen – the substance that gives skin its plumpness – compounding the wrinkle issue,’ she explains.

The redness and sensitivity Pursglove experienced ahead of the experiment worsened by two percentage points, thanks to the makeup locking in irritants, exacerbating allergic reactions. Her pores, on the other hand, had increased in size by five per cent, partly due to their being physically clogged with the makeup.

‘Dirt in pores not only makes them more noticeable and thus appear larger but, over time, we think it can actually stretch them,’ explains Dr Williams.

Large pores were also indicative of the fact that Pursglove’s skin had physically aged during the experiment. ‘Older people naturally have larger pores because, with age, elasticity decreases so the structures that support the skin, and keep pores tight, become slacker,’ says Dr Williams.

Although none of us are likely to endure 30 nights without cleansing, this experiment draws some interesting conclusions. At the end of the venture, Pursglove’s skin had aged by around 10 years – a whole decade in just one month. Fortunately, the damage is not long-lasting – the experiment wasn’t long enough. However, the long-term avoidance of cleansing whilst continuing to wear makeup could leave a more permanent mark.

‘The biggest issue is the accumulation of environmental pollutants, which drive the generation of free radicals,’ says Harley Street dermatologist Dr Sam Bunting. ‘These contribute to the breakdown of collagen and elastin, the structures that underpin youthful skin. While these structures deteriorate with age, you don’t want to do anything to speed up that process.’