As the sun rises and sets the amount of UV rises and falls.
This is because as the sun drops in the sky, more UV is filtered out by the atmosphere.
Once the sun drops below 45 degrees the amount of UV drops very quickly.
In summertime you can get 10 – 12 hours of high UV daylight.
In the winter this can be as low as 2 – 3 hours normally either side of noon.
You may need to use a larger dose for sunrise & sunset exposures.