Is ghusl necessary or is wudu sufficient upon waking from sleep?

Ghusl (ritual bath) or Wudu (ablution) is not obligatory merely upon waking up from sleep. Rather, one performs Wudu for prayer if they intend to pray, and performs Ghusl to remove Janabah (state of ritual impurity) if they are in such a state.

Similarly, when intending to sleep, Wudu or Ghusl is not obligatory for that purpose alone, unless one is in a state of minor ritual impurity and intends to pray.

It is, however, recommended to perform Wudu before sleeping, based on what was narrated by Al-Bara' bin Azib (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When you go to bed, perform Wudu as you would for prayer, then lie down on your right side, and say: 'O Allah, I have surrendered my face to You...'"