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.

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.

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.