If an unmarried couple spend the night together under improper circumstances, would that constitute a sin meriting judicial action?

Yes, if there are no extenuating circumstances, a judicial committee would be formed on the basis of strong circumstantial evidence of sexual immorality.​—1 Cor. 6:18.

The body of elders carefully evaluates each situation to determine whether a judicial committee is warranted. For example: Have the couple been pursuing a romantic relationship? Have they been previously counseled regarding their conduct with each other? What circumstances led to their spending the night together? Did they plan ahead to do so? Did they have a choice in the matter, or were there extenuating circumstances, perhaps an unforeseen occurrence or genuine emergency that left them with no choice but to spend the night together? (Eccl. 9:11) What were the sleeping arrangements? Since each situation is different, there may be other relevant factors that the elders will consider.

After the facts are established, the body of elders will determine whether the couple’s conduct warrants judicial action.