Ash-Shaykh Saalih al-Fawzaan, may Allah preserve him, was asked the following question:
There is an individual who prohibits [the people from] oversleeping in Ramadan and says “it is upon a Muslim to be awake and active [in this month] and it is not appropriate for him to oversleep”. So what is your opinion Oh Noble Shaykh?
Yes he should warn from over-sleeping in this Month [i.e. Ramadaan]. I mean sleeping in the day [time] because it makes one lazy in performing acts of obedience [and] perhaps it may cause one to miss the congregation prayer or it may cause the time for [that specific prayer] to pass by. What is requested from a Muslim is that he be energetic in acts of obedience and he sleeps in the night, especially in the first part of it so he can arise in the morning energetic to perform acts of obedience and to participate in them.
[al-Muntaqaa (111/5) Fatwa Number: 167]
Al-Imaam bin Qudaamah, may Allaah have mercy upon him, said:
The night and the day are together 24 hours, so what is neutral is that one sleeps a third of it; which is eight hours. So whosoever sleeps less than that then he is not safe from his body becoming sick and whosoever sleeps more than that then he will become very lazy. If one sleeps more than that [i.e. eight hours] in the night then there is no need for him to sleep in the day. However, if one sleeps less than that then he should make up for it in the day [by taking a nap].
[Muktasar Manhajil Qaasideen Page:67 Print: Ibn Rajab]
AbdulFattaah Bin Uthman