After a sunburn, your body launches a multi-day process of inflammation, repair, and cell replacement that follows a predictable timeline. Pain typically starts within a few hours of sun exposure, peaks at about 24 hours, and gradually fades over the following week as damaged skin peels away. What’s happening beneath the surface, though, is more complex and has consequences that extend well beyond the discomfort.
The First Few Hours: Why Your Skin Turns Red
UV radiation, particularly UVB rays, penetrates the outermost layer of your skin and damages the DNA inside skin cells called keratinocytes. This damage triggers two immediate responses: the cells begin a self-destruct sequence (a controlled process called apoptosis), and your immune system floods the area with inflammatory signals. Blood vessels in the skin dilate to bring immune cells to the scene, which is what produces the redness and warmth you feel.
The DNA damage is specific. UV light creates structural defects in your DNA strands, essentially fusing together building blocks that should remain separate. Your cells can repair some of this damage, but when the damage is too severe, the cell kills itself rather than risk passing on mutations. These self-destructing cells are sometimes called “sunburn cells,” and they’re a hallmark of UV injury visible under a microscope.
At the same time, UV radiation generates reactive oxygen species, unstable molecules that cause additional damage to cell membranes, proteins, and DNA. This oxidative stress amplifies the inflammatory response and contributes to the pain and swelling that build over the next several hours.
Peak Pain and the 24-Hour Mark
Pain from a sunburn starts within a few hours, but your skin continues to get redder and more irritated as the inflammatory cascade intensifies. Redness and pain peak at roughly 24 hours after the burn. This is when the skin feels hottest to the touch and is most sensitive to pressure or friction.
A mild sunburn affects only the top layer of skin and causes redness and tenderness. A more severe sunburn penetrates into the middle layer, producing blisters, a darker tone, and a shiny or moist appearance. When blisters form, it means the damage has gone deep enough to separate the skin’s layers, and fluid collects in the gap. These blisters are a sign of a second-degree burn and take significantly longer to heal.
When Sunburn Becomes Sun Poisoning
Severe sunburns can cause symptoms that go beyond your skin. If you develop a fever, chills, nausea, vomiting, or a headache after heavy sun exposure, you’re dealing with what’s commonly called sun poisoning. This is your body’s systemic response to widespread skin damage, and it can include dehydration from fluid and electrolyte loss through damaged skin.
Mild to moderate sunburn symptoms typically start fading after about three days. Sun poisoning symptoms last longer and are more severe, often beginning with a red rash and progressing to blisters, swelling, and fever. Complications can include skin infections (particularly if blisters break open) and persistent skin changes that remain after the burn heals.
Why Your Skin Peels
The peeling that starts several days after a sunburn isn’t just dead skin flaking off. UV radiation weakens the bonds between cells in the outermost layer of skin by disrupting the protein structures (called corneodesmosomes) that hold cells together. Think of it like mortar between bricks: the bricks themselves may be intact, but the mortar holding them in place deteriorates. As more UV damage accumulates, these connection points essentially scatter from their normal positions, and the skin’s structural integrity breaks down. The result is sheets of skin that separate and peel away.
Over the next week or so, your skin gradually returns to its normal appearance as new cells from deeper layers migrate upward to replace the damaged ones. Severe sunburns with blistering can take several weeks to fully heal.
What UV Does to Your Immune System
One of the less obvious effects of sunburn is temporary immune suppression in the affected skin. UV radiation depletes a type of immune cell called Langerhans cells from the outer skin layer and impairs their ability to migrate normally. These cells are responsible for detecting foreign invaders and triggering immune responses.
When Langerhans cells encounter the debris from dying skin cells after UV exposure, they can become inappropriately activated in ways that suppress rather than boost immunity. UV also converts a natural compound in the skin into a form that has potent immunosuppressive properties, further reducing the skin’s ability to mount normal immune defenses. This is one reason sunburned skin is more vulnerable to infection, and it’s part of why UV exposure increases cancer risk: the immune system becomes less effective at catching and destroying abnormal cells in the damaged area.
Collagen Damage and Premature Aging
Every sunburn activates enzymes called matrix metalloproteinases that break down collagen and elastin, the proteins that keep skin firm and flexible. Even a single dose of UV radiation can stimulate these enzymes, leading to destruction of the two main types of collagen in skin. Over time, repeated activation diminishes the skin’s capacity to repair itself, producing fine wrinkles, sagging, and leathery texture.
This process, called photoaging, is driven by both direct UV damage and the oxidative stress that follows. The chronic low-grade inflammation from repeated UV exposure keeps these collagen-degrading enzymes active, compounding the damage with each burn. Photoaging is distinct from natural aging and accounts for the majority of visible skin aging in sun-exposed areas like the face, neck, and hands.
Long-Term Cancer Risk
The DNA damage from a sunburn doesn’t always get repaired correctly. When mistakes in repair lead to mutations in genes that control cell growth, those mutations can accumulate over years and eventually lead to skin cancer. The risk is cumulative, and it starts early: five blistering sunburns during childhood increase the lifetime risk of developing melanoma by 80 percent, according to research from the University of Colorado Cancer Center.
This is because the self-destruct mechanism that kills severely damaged cells isn’t perfect. Some cells with significant DNA damage survive, carrying mutations forward as they divide. Combined with the local immune suppression that UV causes, these mutated cells have a better chance of escaping detection and growing unchecked.
Managing a Sunburn as It Heals
The most effective window for reducing sunburn symptoms is early, while inflammation is still building. Anti-inflammatory pain relievers like ibuprofen can help reduce redness and pain when taken according to label instructions. Aspirin works similarly but should not be given to children or teenagers due to the risk of Reye’s syndrome.
Cool compresses and moisturizers help with comfort, and staying well hydrated supports your body’s repair process, especially if the burn is widespread. Blisters should be left intact when possible, since the fluid underneath protects the healing skin beneath. If blisters break on their own, keeping the area clean reduces infection risk. Peeling skin should be allowed to shed naturally rather than pulled off, since the new skin underneath may not be fully developed and is more vulnerable to damage.