Final answer:
The epidermis, specifically the stratum corneum, thickens with continued sun exposure due to the keratinocyte response to UV radiation. Excessive exposure can lead to skin damage and the use of sunscreen is recommended to protect the skin.
Step-by-step explanation:
The layer of skin that thickens with continued sun exposure is the epidermis. Specifically, it is the stratum corneum, the outermost layer, that becomes thicker.
This thickening is a protective response to ultraviolet (UV) radiation. Repeated exposure causes cells called keratinocytes, found in the stratum basale and stratum spinosum, to produce more keratin and migrate to the surface, consolidating into the protective stratum corneum.
While this process can offer some defense against UV radiation, excessive sun exposure can lead to skin damage such as sunburn and even skin cancer due to the harmful effects of UV rays on the deeper layers of skin, particularly the dermis, which can lead to blisters and painful skin. Using sunscreen can protect your skin from these harmful effects and prevent this layer from thickening unnecessarily.