The Night Time Adjustment allows the router to consider how you sail your boat at night (perhaps you reef just before night fall) for safety and comfort reasons.
It is a percentage factor, so if you set the value to 70%, this tells the router you expect to sail 30% slower than the base polar for night time sailing.
Note: This value is applied in addition to any existing Polar Speed Adjustments on your base polar.
For example, if your Predefined Polar has an 80% speed adjustment for both upwind and downwind sailing, and you apply a Night Time adjustment of 90%, the final effective speed would be:
90% of 80% = 72%
So during night sailing, your performance would be reduced to 72% of the original polar values.
Some other relevant information relates to the definition of day/night:
"There are different definitions for sunrise/set: 'official', 'civil', 'nautical', and 'astronomical'. Some references if you like:
https://www.esrl.noaa.gov/gmd/grad/solcalc/solareqns.PDF
http://www.edwilliams.org/sunrise_sunset_algorithm.htm
https://en.wikipedia.org/wiki/Sunrise_equation
The nautical definition is used for the router.
TIP : If using AI Polars
If you use the AI Polars feature, please set your night time adjustment to 100%, as the machine learning algorithm already accounts for night time sailing and automatically applies it to your routing.