During the resting time of a courtship, partners do not have liberty to act as if they were not courting. Even at night, if the moon is shining, it is quite easy to see where you are going. Darkness (sin) allowed in either partner when they are not together is just as bad as allowing it when they are together. Such allowance of sin is sure to have harmful effects on a courtship.
Remember that you can't be more spiritual in your courtship with your partner (day), then what you are spiritually when you are alone (the night) with God. What a person is in public forms their reputation, but what they are in private with God, forms their character. Being the same whether together or apart creates confidence and trust in each partner—a vital ingredient for a happy, fruitful courtship.