Sun and acne: why it improves in summer and worsens in September

It happens to almost everyone living with acne: after a week or two at the beach, their skin seems renewed. Fewer pimples, an even skin tone, less noticeable pores. Then September arrives and, within a few weeks, blemishes return, more numerous and more inflamed than before. It’s not bad luck or a coincidence: it’s called the rebound effect, and it’s a well-documented phenomenon in dermatology. In this article, we’ll look at why the sun seems to improve acne, what really happens beneath the skin’s surface, and, most importantly, what to do now - in the middle of summer - to avoid finding your skin in revolt come September.

Article by Elisa Avalle, founder of LeLang Skin Care and dermocosmetics specialist. Cover image generated with the help of artificial intelligence.

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Why the sun seems to improve acne

Summer improvement in acne is real, but temporary and partly illusory. Three main mechanisms are behind it. The first is the immunomodulatory action of UV rays: sun exposure temporarily reduces the skin’s local inflammatory response, so papules and pustules “calm down” more quickly. The second is the tan itself, which evens out skin tone and optically masks redness and post-acne marks. The third is the mildly disinfecting and sebum-regulating effect of the sun, salt, and seawater, which can dry out surface lesions during the first few weeks.

The problem is that none of these effects address the causes of acne: excessive sebum production, abnormal follicular keratinization, and the proliferation of Cutibacterium acnes. The sun merely hides the symptoms for a few weeks, while setting the stage deep down for an autumn flare-up. If you want to understand the mechanisms of acne in detail, we’ve covered them in our complete guide to understanding acne.

What is the rebound effect, and why does it happen in September?

L’rebound effect (literally, “bounce back”) is an acne flare-up that typically appears between late August and October, after a summer of apparent improvement. The explanation lies in how the skin defends itself against UV rays.

L’thickening of the stratum corneum

To protect itself from the sun, the skin triggers reactive hyperkeratosis: it thickens the outermost layer of the epidermis. This is a serious problem for acne-prone skin, because a thicker stratum corneum blocks follicle openings. Sebum, which continues to be produced (in fact, heat increases its fluidity and quantity), gets trapped. This leads to the formation of countless invisible microcomedones during the summer: a ticking time bomb that explodes in September, when the stratum corneum returns to its normal thickness and inflammation starts up again.

Lipid oxidation

There's also a subtler mechanism that has been studied for years in dermocosmetic research: UV rays oxidize squalene, one of the main components of human sebum. The resulting squalene peroxides are comedogenic and pro-inflammatory molecules: they irritate the follicle wall and promote the formation of new comedones. In practice, the more oily skin is exposed to the sun without protection, the more "aggressive" its own sebum becomes.

Remember: summer acne improvement is a loan, not a gift. Your skin collects on it in September, with interest: that's why rebound prevention starts in summer, not when the pimples have already returned.

Summer mistakes that make rebound worse

Some very common vacation habits amplify the phenomenon. Here are the main ones:

• Spending time in the sun without sunscreen "to dry out pimples": this increases skin thickening and sebum oxidation, exactly the two drivers of rebound.
• Stopping your anti-acne routine because "your skin looks fine anyway": without sebum regulation and exfoliation, microcomedones accumulate unchecked.
• Cleansing aggressively after swimming in the sea and sweating: this depletes the skin barrier and triggers compensatory sebum production.
• Using comedogenic sunscreens or overly rich textures that clog already-stressed pores.
• Touching and squeezing blemishes on tanned skin: the risk of post-inflammatory marks is much higher when melanin is active.

The summer anti-rebound routine, step by step

The good news is that rebound effects can largely be prevented with a lightweight but consistent routine. There’s no need to change everything: just five targeted steps.

1. Purifying yet gentle cleansing

Morning and evening, cleanse to remove sebum, sweat, saltwater, and sunscreen residue without damaging the skin barrier. A purifying mousse with sebum-regulating active ingredients is ideal for oily, blemish-prone skin in summer: it cleanses thoroughly while leaving skin soft, not tight.

2. Daily sebum regulation

This is at the heart of prevention: keep ingredients like Niacinamide, Salicylic Acid, and Zinc in your routine throughout the summer to regulate sebum production, keep follicles clear, and control inflammation. A lightweight serum in the evening is perfect, even in hot weather. If you’d like to learn more about how these active ingredients work together, we discuss it in our guide to Niacinamide and Salicylic Acid for Acne-Prone Skin.

3. Lightweight hydration, even if you have oily skin

This is the step people with acne most often skip, convinced that moisturizing means making their skin greasy. In reality, the opposite is true: dehydrated skin responds by producing more sebum, and in summer, between sun, saltwater, and air conditioning, the skin barrier quickly becomes depleted. You need a lightweight, mattifying moisturizer with active ingredients that normalize sebum production instead of simply absorbing it: Azeloglycine, Zincidone, and Niacinamide work in exactly this way, while also evening out skin tone and the marks left by blemishes.

4. A purifying mask once a week

Once or twice a week, a clay mask helps draw out excess sebum and keep pores clean, preventing the buildup of summer microcomedones. Green clays, combined with soothing and sebum-regulating ingredients, purify without leaving skin dehydrated: an important balance, because "stripped" skin responds by producing even more sebum.

5. Non-comedogenic sunscreen, every day

The one step that can make all the difference in September: an SPF 50 with a lightweight texture, formulated not to clog pores, protects against UV rays that thicken the stratum corneum and oxidize sebum. It also helps prevent post-inflammatory marks on active lesions, which are much more common in summer.

For acne-prone skin, choosing the right sunscreen makes a real difference: new-generation chemical filters in fluid formulations are generally the most comfortable, while denser mineral formulas can feel occlusive. Learn more in our guide to chemical and physical sunscreen filters.

What if the rebound has already started?

If you're reading this article in September and your blemishes have already returned, don't panic: the strategy changes very little; just add one more step. Here are the steps in order:

1. Resume (or continue) using a purifying cleanser and a sebum-regulating serum, morning and evening.
2. Add a gentle exfoliant once or twice a week with Mandelic Acid and enzymes: it helps normalize cell turnover and unclog blocked follicles, and is very well tolerated even by reactive skin.
3. Keep using SPF, even in the city: September UV rays are enough to leave marks from post-acne spots.
4. Don't irritate your skin with physical scrubs or alcohol: this will make the inflammation worse.

If your acne appeared or returned in adulthood, we also recommend reading about acne in your 30s and 40s and how to treat it without antibiotics. For a complete routine for oily skin, check out our guide to a routine for oily skin and enlarged pores.

When to see a dermatologist: if your acne is nodular-cystic, painful, or leaves scars, skincare alone isn’t enough. In these cases, cosmetic treatment should be accompanied by a dermatological assessment.

In summary

The sun doesn’t cure acne: it puts it on hold while, beneath the surface, a thickening of the stratum corneum and oxidation of sebum set the stage for a September flare-up. The difference between a calm autumn and a full-blown rebound comes down to five simple steps you can take now: purifying cleansing, consistent sebum regulation, lightweight hydration, a weekly clay mask, and daily non-comedogenic sunscreen. Your September skin will thank you.