Short, clear answer

• If by “nightfall” you mean eating or drinking after sunset: It is not called a “sin” by itself, but for a shravak during Paryushan it is a clear lapse of vow. The ideal practice is to finish all food and water before sunset (chauvihar). If a lapse happens, do pratikraman with sincere repentance and be more careful the next day. Both Shwetambar and Digambar traditions uphold daytime-only eating during these days.

• If by “nightfall” you mean an involuntary nocturnal emission (wet dream): Jain dharma judges mainly by intention (bhava). An involuntary occurrence is not a deliberate transgression. Maintain purity in thoughts, avoid triggers, keep brahmacharya, and if there was any lapse in thoughts or conduct, do pratikraman. Focus on inner vigilance rather than guilt.

Helpful references

• Daylight-only eating during Paryushan:
• Water/food after sunset (chauvihar):
• Daily pratikraman for any lapse:
• Brahmacharya as part of conduct in Paryushan: