While cruising, you most often decrease your performance at night: no spinnaker, safety reef, etc… enter 10% and the polar efficiency speed will be decreased by 10% on top of the low and high wind efficiency parameters.
For example, if you have a 80% low wind parameter and a 10% night efficiency loss. Your overall efficiency at night will be 70%. SailGrib automatically calculates the sunrise and sunset times depending on the position of the boat during the weather routing algorithm.