A good night’s sleep isn’t just about the number of hours you log. Getting quality sleep — the kind that leaves you feeling refreshed and ready for the day — is critical for a healthy brain.

People with disturbed sleep, like insomnia or sleep apnea, have a higher risk of developing dementia than those with no sleep issues. Poor sleep can harm your brain in other ways, too. One study found that people in their 30s and 40s with heavily disrupted sleep (such as frequent awakenings or movements) were two to three times more likely to test lower in executive function, working memory and processing speeds a decade or so later.

Scientists think that deep sleep and rapid eye movement (or REM) sleep are particularly influential when it comes to brain health and dementia risk. A study published last month on people with deep sleep and REM deficiencies found that the subjects’ brains showed signs of atrophy in M.R.I. scans 13 to 17 years after the deficiencies were observed; the atrophy looked similar to what you’d find in the early stages of Alzheimer’s disease.

What Scientists Know So Far

When you’re asleep, your brain continuously cycles through four distinct phases: Two stages of lighter sleep, when your body relaxes and your heart rate and temperature drop; deep sleep or slow wave sleep, when brain activity slows; and REM, when you typically dream. The brain generally takes about 90 minutes to cycle through all four stages and then restarts the process.

Deep sleep and REM help your brain “heal itself” from fatigue and stress and consolidate memories, said Matthew Pase, an associate professor at the School of Psychological Sciences at Monash University in Melbourne, Australia. In deep sleep, your brain regulates metabolism and hormones; it also acts as a “rinse” for the brain, clearing out waste. REM is when your brain processes emotions and new information you picked up when you were awake.

The two phases influence dementia risk in different ways, scientists think.

As part of the rinsing process in deep sleep, your brain flushes out amyloid proteins that are a hallmark of Alzheimer’s. Years of interrupted deep sleep and incomplete flushing — known as glymphatic failure — could hasten the onset of dementia, said Dr. Maiken Nedergaard, a neurology professor at the University of Rochester Medical Center who researches the glymphatic system.