What is the ruling on fasting six days of the month of Shawwal during the night shift, which begins at 6:00 PM and ends at 6:00 AM, knowing that most of the day is spent sleeping?

There is no harm in fasting, and sleeping most of the day does not invalidate it. If a fasting person sleeps the entire day after having made the intention to fast during the night, their fast is valid according to the majority of scholars. They are in agreement that the fast is valid if the person woke up for a moment and then slept the rest of the day.