Tanning is the biological process where skin darkens in response to ultraviolet (UV) light exposure. This darkening occurs as the body attempts to protect itself from radiation damage. While the sun’s intensity changes drastically throughout the day, it is absolutely possible to get a tan around 6 PM. The mechanism behind this late-afternoon tan differs significantly from the process that happens at midday, primarily due to the specific types of UV radiation that persist as the sun sets.
The Science of Skin Tanning
The skin’s natural defense against sun damage is the production of a pigment called melanin. Specialized cells in the outer layer of the skin, known as melanocytes, are triggered by ultraviolet radiation to initiate this process. Melanin is a dark substance that absorbs UV light, effectively forming a protective shield over the deeper skin layers. The more UV exposure the skin receives, the more melanin is produced and distributed to the surface cells, which results in the visible darkening we call a tan.
The body exhibits two distinct tanning phases in response to UV exposure. The first is Immediate Pigment Darkening, which is a quick oxidation of existing melanin, causing a temporary color change. The second is the Delayed Tanning Response, where new melanin is manufactured over the course of days, providing longer-term protection against future damage.
Understanding UV Levels After Peak Hours
The dramatic reduction in the sun’s power around 6 PM is explained by a physical principle: the Solar Zenith Angle. This term describes the angle of the sun relative to a point on the Earth’s surface. As the sun drops closer to the horizon in the late afternoon, its rays must travel a significantly longer path through the atmosphere. This increased distance through the air allows the atmosphere to absorb and scatter a far greater amount of the ultraviolet radiation.
Specifically, the shorter UV wavelengths are subjected to more scattering. The result is a much lower total UV intensity, which is reflected in the UV Index reading. While a midday UV Index might be high (8+), the late-afternoon reading typically drops to a low (1-2) or moderate (3-5) rating. A practical indicator of this reduction is the “shadow rule,” which suggests UV exposure is lower when your shadow is taller than you are.
The Role of UVA Versus UVB at Dusk
Sunlight that reaches the Earth is composed of two main types of ultraviolet radiation: UVB and UVA. These two types behave very differently as the day progresses because of their differing wavelengths. UVB rays are shorter, more energetic, and are the primary cause of sunburn and the body’s delayed, long-lasting tanning response. Because UVB rays are easily scattered by the atmosphere, their intensity drops off dramatically as the sun approaches the horizon, meaning they are nearly absent by 6 PM.
UVA rays, however, have a longer wavelength, allowing them to penetrate deeper into the skin’s dermal layer. This deeper penetration means UVA is strongly associated with long-term damage like premature aging, fine lines, and wrinkles. Crucially, the intensity of UVA radiation remains relatively consistent throughout all daylight hours, even when the sun is low in the sky. This persistence means the late-day tan is overwhelmingly a result of UVA exposure, which causes the immediate darkening of existing melanin pigment.
Consequently, the ratio of UVA to the less intense UVB is at its highest during the early morning and late evening. This dominance of persistent UVA is the scientific reason a person can still achieve a tan at 6 PM. In fact, many commercial tanning beds utilize high percentages of UVA to deliver color without the immediate burning risk associated with peak-hour UVB.
Safe Exposure Guidelines for Late Afternoon
The belief that late-afternoon sun is entirely harmless is a common misconception that overlooks the persistent UVA rays. Since UVA contributes to photoaging and increases the risk of skin cancer, protection remains necessary. It is important to check the local UV Index, as protection is recommended whenever the reading is 3 or higher, which can last past 6 PM in summer months.
Applying a broad-spectrum sunscreen with an SPF of at least 30 is necessary, as this formula protects against the deeper-penetrating UVA rays present all day. If spending an extended period outdoors, reapplication every two hours is still necessary. Wearing protective clothing, such as a wide-brimmed hat or UPF-rated garments, offers another layer of defense against cumulative exposure. Furthermore, the low angle of the setting sun can cause intense horizontal glare, making quality sunglasses necessary for eye protection.