Hmm, you bring up a very interesting point. Personally, I don’t know anyone who sleeps on their stomach, perhaps because the prone position is physiologically quite an unnatural posture to preserve. There is also a lack of research on this front besides tummy-sleeping in babies, which is a huge concern because of its high association with mortality (SIDS). If I could chime in a wild guess for a correlation between a prone sleeping position and brain disease, it would be directed towards a bridging condition called obstructive sleep apnea, in which someone’s lungs are physically restricted to expand during sleep (due to the weight pressing down on the lungs), causing hypoxia (low oxygen in the blood), leading to low oxygenation in the brain. Obstructive sleep apnea is known to cause certain types of brain diseases as I mentioned in my article. Hope this helps!