The night time adjustment is applied on top of the upwind/downwind adjustment and allows the router to consider that you sail the boat slower/with less sail at night (perhaps you reef just before night fall) for safety reasons. It is a percentage factor, so if you use the value 70%, this tells the router you expect to sail 30% slower than the base polar you have set. Please note: This value will be added in addition to any other Polar Speed Adjustments you have applied to your base polar.

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.