Each hanyou(half-demon) has a time when they lose their youkai(demon) powers and are rendered a mere mortal. For Inuyasha, his time of vulnerability is during the night of the new moon. When Inuyasha transforms, it's after the sun sets on the night of the new moon and reverts back to his hanyou state when the sun rises. All hanyous are different--not all transform during the night, some transform during the day.