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 seaside, the skin seems reborn. Fewer spots, a more even complexion, less visible pores. Then September arrives and, within a few weeks, the blemishes return, more numerous and more inflamed than before. It is neither bad luck nor a coincidence: it is called the rebound effect, and it is 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, above all, what to do now — in the middle of summer — to avoid finding yourself with skin in revolt come September.

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Why the sun seems to improve acne

The summer improvement in acne is real, but it is 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 subside more quickly. The second is the tan itself, which evens out the complexion and optically masks redness and post-acne marks. The third is the mildly disinfecting and sebum-regulating effect of sunlight, salt and seawater, which can dry out superficial lesions during the first few weeks.

The problem is that none of these effects addresses the causes of acne: excess sebum production, abnormal follicular keratinization and the proliferation of Cutibacterium acnes. The sun merely hides the symptoms for a few weeks while, deeper down, it prepares the ground for the autumn flare-up. If you want to understand the mechanisms of acne in detail, we have collected them in the complete guide to understanding acne.

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

The rebound effect is the 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.

Thickening of the stratum corneum

To protect itself from the sun, the skin activates reactive hyperkeratosis: it thickens the outermost layer of the epidermis. On acne-prone skin, this is a serious problem because a thicker stratum corneum blocks the openings of the follicles. Sebum, which continues to be produced (in fact, heat increases both its fluidity and its quantity), becomes trapped. During the summer, countless invisible microcomedones therefore form: a time bomb that explodes in September, when the stratum corneum returns to its normal thickness and inflammation starts again.

Sebum oxidation

There is 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 squalene peroxides that result are comedogenic and pro-inflammatory molecules: they irritate the follicular 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. The skin pays it back in September with interest: that is why rebound prevention should begin in summer, not when the spots have already returned.

Summer mistakes that worsen the rebound effect

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

• Exposing yourself without sunscreen “to dry out spots”: this increases stratum corneum thickening and sebum oxidation — exactly the two drivers of the rebound effect.
• Stopping your anti-acne routine because “the skin looks fine anyway”: without sebum regulation and exfoliation, microcomedones accumulate undisturbed.
• Cleansing too aggressively after swimming in the sea and sweating: this weakens the skin barrier and stimulates 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 anti-rebound summer routine, step by step

The good news is that the rebound effect can largely be prevented with a lightweight but consistent routine. There is no need to overhaul everything: you need five targeted steps.

1. Purifying but gentle cleansing

Morning and evening, cleanse in a way that removes sebum, sweat, salt and sunscreen residue without damaging the barrier. A purifying mousse with sebum-regulating active ingredients is ideal for oily and blemish-prone skin in summer: it cleanses thoroughly while leaving the skin soft, without that tight feeling.

2. Daily sebum regulation

This is the heart of prevention: keep ingredients such as Niacinamide, Salicylic Acid and Zinc active throughout the summer. They regulate sebum production, keep follicles clear and help control inflammation. A lightweight serum in the evening is perfect even in hot weather. If you want to learn more about this combination of active ingredients, we discussed it in our guide to Niacinamide and Salicylic Acid for acne-prone skin.

3. Lightweight hydration, even if your skin is oily

This is the step most often skipped by people living with acne, who believe that moisturizing means making the skin greasy. In reality, the opposite is true: dehydrated skin responds by producing more sebum, and in summer, between sun, salt and air conditioning, the barrier quickly becomes depleted. You need a lightweight, mattifying cream with active ingredients that normalize sebum rather than merely absorbing it: Azeloglycine, Zincidone and Niacinamide work in precisely this way, while also evening out the complexion 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, counteracting the buildup of summer microcomedones. Green clays, combined with soothing and sebum-regulating active ingredients, purify without leaving the skin dehydrated: an important balance, because skin that feels “stripped” responds by producing even more sebum.

5. Non-comedogenic sun protection every day

This is the one step that can change the course of 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 prevents post-inflammatory marks on active lesions, which are much more common in summer.

When it comes to acne-prone skin, the choice of sunscreen makes a real difference: newer-generation chemical filters in fluid formulas are generally the most comfortable, while denser mineral textures can feel occlusive. We discuss this in detail in the guide to chemical and physical sunscreens.

What if the rebound effect has already arrived?

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

1. Resume (or continue) using a purifying cleanser and sebum-regulating serum, morning and evening.
2. Introduce gentle exfoliation once or twice a week with Mandelic Acid and enzymes: this helps normalize cell turnover and clear blocked follicles, with excellent tolerance even on reactive skin.
3. Continue using SPF in the city as well: September UV rays are enough to darken post-acne marks.
4. Do not attack the skin with physical scrubs or alcohol: this would worsen inflammation.

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

When to see a dermatologist: if your acne is nodular or cystic, painful or leaves scars, skincare alone is not enough. In these cases, a cosmetic routine should be combined with a dermatological evaluation.

In summary

The sun does not cure acne: it puts it on hold while, beneath the surface, stratum corneum thickening and sebum oxidation prepare the September flare-up. The difference between a calm autumn and a full-blown rebound effect is decided now, with five simple steps: purifying cleansing, consistent sebum regulation, lightweight hydration, a weekly clay mask and non-comedogenic sun protection every day. Your September skin will thank you.