What is the ruling on the fast of one who sleeps most of the day during the fasting hours of Ramadan?

Sleeping for most of the day during the fasting hours of Ramadan does not invalidate the fast. In fact, even if a fasting person were to sleep for the entire day, their fast would still be valid. However, they would be depriving themselves of a great reward. Imam al-Nawawi stated in Rawdat al-Talibin wa ‘Umdat al-Muftin (2/366), "If one were to sleep for the entire day, one’s fast would still be valid as is well known and established." Similarly, Imam Ibn Qudamah wrote in Al-Mughni (3/116),

However, Maliki scholars have maintained that excessive sleep during the fasting hours of Ramadan is disliked (makruh). Al-Ḥaṭṭāb stated in Mawahib al-Jalil fi Sharh Mukhtasar Khalil that excessive sleep during the daylight hours is disliked as transmitted by Qadi Iyad and Ibn Jawzi.

There is no doubt that the reward of a fasting person is proportional to the effort and hardship endured during the fast; the Prophet (peace and blessings be upon him) told Aisha (may Allah be pleased with her),

And Allah Almighty knows best