What is the best time to sunbathe?

Tanning refers to the skin pigmentation that occurs as a result of sun exposure. From a biological perspective, UV rays stimulate the synthesis of melanin by melanocytes, which are cells in the epidermis. This is a defence mechanism designed to limit the harmful effects of UV rays on the skin, which can accelerate skin sagging and cause hyperpigmentation, sunburn , or skin cancers.

Melanin operates by enveloping the nucleus of keratinocytes, the cells of the stratum corneum, where it forms a filter that protects DNA from the mutagenic and carcinogenic effects of UV rays. This pigment is capable ofabsorbing approximately 50% of UVA and 85% of UVB that reach the skin. Melanin can also capture free radicals generated within the body by UV radiation, thus limiting the premature ageing of the skin.

Important : While the process of melanogenesis occurs to protect the skin from UV rays, it does not exempt the need to use a broad-spectrum sunscreen on areas of the body exposed to the sun, and to reapply every two hours.

Indeed, sunbathing requires sunlight, but it should not be too intense. In fact, between 11 am and 4 pm, when solar radiation is at its most energetic, the skin is more likely to burn and suffer from sunburn than to tan.

Thus, to limit skin dryness while tanning, it is recommended to expose oneself to the sun in the morning, between 9:30 and 11:00, or in the late afternoon, between 16:00 and 19:00. It should also be noted that it is not advisable to have more than one tanning session per day, as it is more effective and safer to tan gradually.

It is possible to tan in any season, even in winter. In fact, the presence of snow makes tanning more effective, as it is capable of reflecting up to 85% of UV rays. However, between the ski suit, protective glasses and hat, there is a high risk of ending up with a rather patchy tan.

Tanning sessions are therefore usually when the weather is fine, typically between late spring and early autumn. However, it is crucial to exercise extra caution during the summer, when the sun's rays are hotter and more intense, and to never forget your hat and sunscreen. If UV rays are more energetic in the summer, it's due to the Earth's tilt angle, which does not rotate vertically around the sun but is slightly inclined (≈ 23°). The sun's rays therefore strike either the North or South Pole depending on the time of year, which explains why the seasons are "reversed" at each pole.

Finally, if you observe a few clouds in the sky, rest assured, this will not prevent you from tanning. Indeed, only large dark clouds are capable of blocking all UV rays. A mid-altitude cloud layer can only stop 30 to 60% of the sun's rays. Furthermore, if the clouds are white and scattered, the UV rays will reflect and the radiation will then increase by 10%.

• ANANTHASWAMY H. Toxic effects of ultraviolet radiation on the skin. Toxicology and applied pharamcology (2004).

• SITUM M. & al. UV-radiation, apoptosis and skin. Collegium Antropologicum (2011).