Also known as 'sleep glycolics' these new serums and toners work hard while you're spark out. Here's how to use them and our top picks

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If you want to boost your beauty sleep and wake up glowing, then the latest skincare step to know about is the 'sleep peel' or 'sleep glycolic'. A raft of these overnight exfoliators have hit our shelves in recent weeks. This month alone we've said hello to  Medik8 Sleep Glycolic , £35, and  Bad Habit Power Sleep Melatonin Glycolic Night Cream , £25, which followed on from the blockbuster launch last year of  Alpha H Liquid Gold Midnight Reboot Serum , £70.

Why are they top of our list right now? “Using glycolic acid overnight means you’ll wake up with an instantly brighter complexion and considerably smoother feeling skin,” explains Medik8’s director of research, Daniel Isaacs.

What does glycolic acid do and why is it a creature of the night?

Glycolic is an  alpha hydroxy acid  (AHA) that is especially helpful if you’re trying to create an even skin tone, blast dullness and refine your skin texture.

“Glycolic acid breaks down the bonds between skin cells to encourage cell turnover," explains Daniel. “It sloughs off dead skin cells to reveal a visibly smooth, glowing complexion. Pores are visibly minimised, skin is decongested and the appearance of  hyperpigmentation  is lifted and clarity of the skin is improved.”

Glycolic acid also has ultra-effective penetration, as it has the smallest molecular size of all exfoliating acids. This means it can work more deeply than other AHAs such as  lactic aci d, which has a larger molecular size and acts more effectively on the surface (it's a dab hand at helping skin retain moisture). As a result, glycolic acid can address more stubborn skin concerns such as advanced signs of ageing and hyperpigmentation.

MORE GLOSS: A guide to using exfoliating acids in your skincare routine

Why should glycolic acid be used overnight?

If you've used a glycolic, you'll see it mainly occurs in overnight skincare such as  night creams.  This is because deep-acting glycolic acid can increase your photosensitivity, making you more susceptible to sun damage and pigmentation. Using it overnight reduces this risk; regardless of this, if you’re using acids in your skincare you should apply  SPF  the day after application.

How strong should my glycolic acid be?

Glycolic acids come in different percentages, for example, Alpha H’s Liquid Gold Midnight Reboot Serum has 14 per cent glycolic acid, while MediK8’s Sleep Glycolic has 10 per cent.

Skincare expert Dija Ayodele advises not to get too hung up on percentages. "Your own skin will play a big part in how you tolerate it," she tells us. "Ten per cent and is fine for most people, but if someone has compromised skin, that might be irritating for them." She advises always doing a patch test before using a new glycolic acid serum, or toner "regardless of percentage".

Can you always leave glycolic acid on overnight?

No, it needs to have been formulated specifically as a sleep peel or overnight glycolic (we've selected the best of these below). "For a glycolic acid to be left on overnight it needs to have been formulated to be slow-release so it’s not too harsh on the skin,” says Dija.

So while we loved glycolic newcomers  No7   Laboratories' Resurfacing Peel 15% Glycolic Acid , £17, and  Charlotte Tilbury’s Super Radiance Resurfacing Facial , £52, these have to be removed after five minutes (though Charlotte Tilbury says that as you build up resilience to hers, you can leave it on for up to 20 minutes).

The best overnight glycolic acids and sleep peels to buy now

Medik8 Sleep Glycolic, £35

Designed to be used twice weekly, this creamy fragrance-free serum uses ten per cent glycolic acid to brighten, smooth and decongest the skin. It is ideal for those who are new to chemical peeling because the glycolic acid is encapsulated in a shell that is slowly broken down when it comes into contact with our skin enzymes. The encapsulation means the acid works gradually overnight to deliver the exfoliating powers without the risk of irritation.

Other ingredients include glycerin to draw moisture to the skin to make it hydrated and supple and Korean clover extract to support skin repair and the natural skin barrier. Applying this post-evening cleanse to face, neck and décolletage then following with serum and moisturiser. The next morning do your normal skincare routine with SPF. Not suitable for very  sensitive skin .

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Bad Habit Power Sleep Melatonin Glycolic Night Cream, £25

If you're not yet acquainted with Bad Habit, the bold skincare range from the same umbrella brand as Morphe and Morphe 2, this purple-coloured cream is the perfect introduction to the no-nonsense, high-performing range which also includes cleansers and a mist, among others. This cream aims to soothe stressed-out skin that's had a bit of a battering (late nights, junk food). The idea is it will erase your bad habits! It combines 3.5 per cent glycolic with hyaluronic acid  and melatonin , which supports your skin’s nighttime antioxidant production to help fight damaging free radicals such as UV exposure and pollution.

It's subtly scented with lavender and chamomile which makes it a soothing nighttime ritual and is light enough to absorbs quickly and you don't feel you're leaving half the cream on your pillow. We've been using this for about a week and can definitely see an improvement in skin clarity.

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Alpha H Liquid Gold Midnight Reboot Serum, £70

Alpha H was a pioneer of glycolic acid in skincare when it launched  Liquid Gold,  £30.25, way back in 1995. The brand has now introduced this hydrating night serum to the family as an overnight treatment. It has 14 per cent glycolic acid in it with added granactive retinoid, antioxidants, coenzyme Q10, peptides and nourishing vitamin E and jojoba extract. It feels rich with a touch of light facial oil about it. Moisturising and multitasking this is a serum our editorial director Victoria Woodhall  uses to the very last drop .

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Drunk Elephant T.L.C. Framboos Glycolic Night Serum, £76

This week saw  Drunk Elephant announce its first serum  in eight years and it's in good company alongside this one, (although don't use the two together). This has a gel formula that sinks in like a dream, combining ten per cent glycolic acid with tartaric, lactic, citric and spot zapping salicylic to improve the appearance of lines and uneven skin tone. Drunk Elephant recommends applying this on the face, neck, chest and the backs of your hands at night.

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Skinceuticals Glycolic 10 Renew Overnight Corrective Cream, £80

This radiance-enhancing cream combines ten per cent glycolic acid with two per cent phytic acid which is famed for its skin-detoxifying powers. There's a soothing complex in the mix too to minimise dry patches and prevent inflammation to support the skin barrier, making this a good choice for those with more sensitive skin (although do always patch test).

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The Ordinary Glycolic Acid 7% Toning Solution, £6.80

This toning water contains seven per cent glycolic acid for a mild exfoliating effect plus Tasmanian pepper berry derivative, an ingredient known to help reduce the irritation that sometimes comes with acid use. There's also ginseng and  aloe vera  to soothe the skin. Saturate a cotton pad with this and swipe it over your skin. No need to rinse.

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Sunday Riley Good Genes Glycolic Acid Treatment, £85

This cult Sunday Riley serum feels luxuriant and fuses lactic acid with seven per cent exfoliating glycolic acid to provide extra exfoliation with hydrating prickly pear in the line-up too to reduce irritation. Lemongrass extract helps the acids in their quest to brighten and clarify the skin too. GTG's commercial projects manager Catherine applies this before bed as instructed to wake up with brighter, cleaner, fresher looking skin. She notes that it's a tad tingly on application but nothing her sensitive skin can't tolerate.

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Summer Fridays Soft Reset AHA Exfoliating Solution, £49

Summer Fridays is best known for the ultra-hydrating Jet Lag mask , £22.50, but we implore you to try this three-in-one overnight treatment too. It has 16 per cent glycolic acid to address dull, uneven skin and enlarged pores, as well as niacinamide  to clarify the skin's texture. It has aloe vera in too to hydrate the skin and soothe. The brand recommends using this every night, but our tester experienced a little irritation and found three times a week better.

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Bioderma Sebium Night Peel, £16.50

This overnight peel which launched this month, was formulated to aid oily and acne-prone skin. It mixes 15 per cent glycolic acid with B vitamin d-panthenol which has a soothing action and Bioderma's Fluidactiv, a mattifying ingredient found in other Bioderma skincare including Bioderma Sebium Hydra  moisturiser, £11.50, that helps to avoid clogged pores.

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The Inkey List Glycolic Acid Liquid Toner, £6.99

The Inkey List  is known for high-performing budget buys and this glycolic acid toner fits that brief. The ten per cent glycolic acid in this is paired with five per cent witch hazel to minimise oil and blemishes. As with any toner this should be applied on a cotton pad (only of an evening) and not rinsed off. The Inkey List recommends starting with using this every other night and increasing to once daily when you know how your skin tolerates it

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MORE GLOSS: Are skin peels right for you? A skincare expert explains