A study performed in 2010 in Australia compared two groups of adults for five years. Half of the participants applied sunscreen when they felt it was necessary. The other half applied sunscreen on a daily basis. The study participants who used sunscreen daily had half the amount of melanoma of those who applied sunscreen when they wanted.
UVB lights are the rays responsible for sunburn and previously thought to be the only rays of importance. However, the rationale behind the difference in applying daily vs. when needed is that UVA light may play a larger role than anticipated. UVA light is present even under cloud cover, which is why it is important to apply sunscreen daily for full skin coverage.
Consistent application of daily sunscreen is necessary to help protect skin. Newer sunscreens block both UVA and UVB rays, see if your daily sunscreen passes the test.