This kind of sleep is essential for a healthy brain

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.

- Mohana Ravindranath

People with disturbed sleep, like insomnia or sleep apnoea, 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 in March on people with deep sleep and REM deficiencies found that the subjects' brains showed signs of atrophy in MRI 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.