https://www.newscientist.com/article/2183502-skin-tans-the-most-when-spending-every-other-day-out-of-the-sun/

Sunlight contains ultraviolet (UV) rays that can damage DNA and lead to premature ageing and skin cancer. To protect against this, our skin produces melanin, a dark pigment that acts like a natural sunscreen. The pigment starts to form within hours of sunlight exposure and gives the skin a tanned look.

Carmit Levy at Tel Aviv University in Israel and her colleagues measured how much melanin mice produced when they were exposed to UV light every day, every second day, or every third day.