Assallamualeykum! Why is it bad to sleep face down? Are the hadiths about it reliable?
There appears are two narrations that mention the Prophet disliking sleeping on the stomach, one in the collection of al-Tirmidhī and one in the collection of Ibn Māja. The imams al-Bukhāri, al-Dāraquṭnī, Ibn Abī Ḥātim and Ibn Rajab consider these narrations unreliable, while al-Albānī considers them reliable. So it is not firmly established that it is disliked or forbidden to sleep on the stomach. Scholars like Ibn al-Qayyim mention that it is harmful to one’s health to sleep on the stomach.
A fatwa by the Jordanian fatwa authority considers it disliked (makrūh), meaning that it is not forbidden (ḥarām). The fatwa also says that if a person has to sleep in that way for health reasons, or if they turn onto their stomachs while asleep (without intending it), then it is not disliked.
According to many English-language health sites children younger than 12 months should not sleep on the stomach due to the increased danger of SIDS (Sudden Infant Death Syndrome).