Praise be to Allah the Lord of the Worlds.
According to jurists, it is reprehensible to sleep lying down on one`s belly. The evidence on this is the narration in which the Messenger of Allah (PBUH) said: "Lying down this way is disapproved by Allah" {Abu Dawoud}.
In some narrations: "Lying down this way is characteristic of the people of Hell." {Ibn Majah}. Listing the people recommended to be woken up from sleep, Ash-Shirbini (May Allah have mercy on him) said: "The person who is sleeping lying down on his/her belly since this is characteristic of the people of Hell." (Moghni Al-Mohtaajj, vol.1/pp.309). Based on the aforementioned, it is reprehensible to sleep lying down on one`s belly.
In addition, some scientific papers have indicated that sleeping in this manner has serious repercussions on the kidneys and the respiratory system.
The above pertains to the one who sleeps in that manner without a legitimate reason. As for the one who does so unintentional, such a person isn`t held responsible, in light of Sharia, because his/her actions aren`t recorded until he/she wakes up. Moreover, the sick who needs to sleep in this manner isn`t to blame since necessity knows no law.
In conclusion, we advise the questioner to follow the Sunnah of the Prophet(PBUH) by sleeping on his/her right side since he (PBUH) told Al-Baraa` Bin Azib to do so: "Whenever you go to bed, perform Wudu' as you do for Salat then lie on you right side." {Related by Bukhari & Muslim}. And Allah The Almighty Knows Best.