Retinoids should always be applied at night.
Retinoids are widely thought to increase skin's sensitivity to the sun. Because the component is light-sensitive, it frequently comes in an opaque tube. However, if you use a retinoid at night, it won't make your skin any more sensitive to UV radiation. The following morning, just put on your usual amount of clean sunscreen, and you'll be protected.
Nobody expected that the first gravitational waves would be observed from a binary black hole system.