While the body sleeps, it heals. Injuries and illnesses all improve when the body gets adequate sleep; so do chronic conditions such as arthritis and fibromyalgia.

Research shows that most of the healing takes place during Stage 2 and Stage 3 sleep, when sleep is deepest and without dreams.

During Stage 3 sleep, healing hormones such as human growth hormone are released and the produc­tion of stress hormones, such as cortisol, is inhibited. Tissue repair happens during this stage. Blood pres­sure drops and blood flows more to the muscles. Inflammation is suppressed so that healing can happen. Stage 3 deep sleep is when muscles and joints are at their most relaxed.

Adequate, high-quality sleep is also crucial for brain health and keeping your memory sharp. Memories are consolidated during sleep as the infor­mation a person learns during the day is transferred from short-term to long-term memory.

The connections between brain cells that help you remember things are strengthened. Long-term memories are formed in the brain during Stage 3 deep sleep, another example of how the quality of sleep is as important as the quantity.

Another cause of poor memory and bad decision-making from lack of sleep are high levels of the stress hormone cortisol. Elevated levels of cortisol have a negative effect on the hippocampus, a structure in the brain that is closely tied to memory and the consolidation of information.

When you get adequate, high-quality sleep, your cortisol levels drop during the night, giving your hippocampus a chance to do its job of organizing memories. When you don’t sleep well, with cortisol levels higher than normal, the result is forgetfulness, difficulty processing new information and trouble concentrating and staying focused.

A lack of good sleep is associated with Alzheimer’s disease and some other forms of age-related dementia. During sleep, the brain’s specialized lymphatic system flushes out meta­bolic waste products that accumulate in the fluid between brain cells. That includes beta-amyloid and tau, the proteins that form the character­istic brain plaques and tangles of Alzheimer’s disease.

We know from earlier studies that acute sleep deprivation elevates beta-amyloid levels in the brain. Recent research has shown that even one night of poor quality sleep is enough to cause an immediate rise in beta-amyloid levels; a week of poor quality sleep raises both beta-amyloid and tau.

Thus, the quality of sleep here is more important than quantity. Reaching stage 3 deep sleep is crucial for letting the brain flush out waste products.