All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The time of standing at ‘Arafah starts at the noon of the ninth of Thul-Hijjah, which is the middle of the day at the time of the Thuhr Prayer, and lasts until the dawn of the Day of Nahr. Any one who stands at any part of this time will have fulfilled standing at ‘Arafah. This is proved by what was narrated in the Sunan that the Prophet, sallallaahu ‘alayhi wa sallam, said to ‘Urwah ibn Mudharris: "Anyone who witnessed our prayer with us and stood with us until we leave Muzdalifah to go to Mina and stood before this at ‘Arafah during the night or day, his Hajj will be complete and his untidiness will be ended."
According to the majority of scholars, it is obligatory for the one who stood during the day to extend this and include part of the night. If the pilgrim did not stand part of the night, he should slaughter a sacrifice because he missed one of the obligations of Hajj. There is no harm to go out of ‘Arafah and return back as standing at ‘Arafah should not necessarily be continuous. Thus, if you stood at ‘Arafah during the day and extended your standing after sunset, your Hajj would be valid. If you left ‘Arafah then returned, this would be alright, but if you left standing at part of the night, you should slaughter a sacrifice according to opinion of the majority of scholars.
Allaah Knows best.