In Jain Dharma, using water after dark is generally avoided because night-time activity can cause hiṃsā to very small जीवs in water or around the area of use.

So for washing hands after dark:

• Traditionally, it is not preferred
• If it can be done before sunset, that is better
• In situations of necessity, different traditions and household practices may be more lenient, but the ideal Jain observance is to minimize use after dark

The main principle is ahimsa and carefulness in all actions, especially at night.