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 your polars 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, which should correspond to when you have enough daylight to use for navigation.

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.