the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
It is not sinful for a fasting person to sleep during the day in Ramadan. It has no affect on the validity of the fast, no matter how much of the day a person sleeps.
It is also by no means encouraged to sleep during the day in Ramadan. This was not the practice of the Companions nor of the Pious Predecessors. If anything, they increased their daytime activities in Ramadan. It is merely permissible to sleep.
There is a weak hadîth that reads: "The sleep of the fasting person is worship." However, this hadîth is far too weak to be used as evidence for anything.
Even if we assume – purely for the sake of argument – that the hadîth were authentic, it still does not justify laziness. It certainly does not pardon the practice of those who sleep all day in Ramadan and then spend the nights in feasting and merriment. The hadîth only refers to a person's normal sleeping habits, like a person's usual daily nap. This normal sleep helps invigorate the person for further acts of worship.
And Allah knows best.
Source: Islam Today