Why Deep Sleep and REM Sleep Are Crucial for Brain Health https://pubmed.ncbi.nlm.nih.gov/40110600/ 2 particular phases seem to play outsize roles in cognitive health. Poor sleep quality could be an early sign of Alzheimer’s https://www.nytimes.com/2025/04/25/well/mind/deep-sleep-rem-brain-health.html Repeated lack of deep sleep and REM ruins you