Rapid eye movement (dreaming) - REM

About 80% of our sleeping is of the SWS variety, identified by slow brain waves, relaxed muscles and deep breathing.

Deep sleep is important for the consolidation of memories. New experiences get moved to long-term storage and less important experiences from the previous day get cleared out.

Dreaming accounts for 20% of our sleeping time.

The length of dreams can vary from a few seconds to almost an hour. During REM sleep, the brain is highly active. The muscles are paralyzed, and the heart rate increases. Breathing can become erratic. 

Although eight hours is the common mention, optimum sleep can vary from person to person and from age to age.

One review that worked through 320 research articles concluded 7 - 9 hours of sleep are enough for adults. According to experts, too little or too much sleep can both have a negative impact on your health.

Our bodies are synchronized with the day-night cycle as our planet rotates. Our circadian rhythms continue even in the absence of any external input. Even plants that are kept in a dark cupboard at a stable temperature open and close their leaves as though they can sense the sun.

We have a gene that releases a protein that builds up in cells overnight and gets broken down in the daytime. The clock gene is active in almost every cell type in the body and under circadian control.

Virtually every activity in our bodies - related to the blood, liver, kidneys and lungs as well as the secretion of hormones and body temperature - is influenced by the time of day they are normally needed.

Poor sleeping habits are not a modern problem.

One study found tribes that don't have access to modern electrical devices to be prone to intermittent sleep. The main difference is that they do not have anxiety about their sleep patterns like western countries do.

A day or two of sleep deprivation can cause healthy people to suffer hallucinations and physical symptoms.

Cognitive abilities are impaired after a poor night's sleep. Concentration and memory are affected, and people are more likely to be impulsive.

Sleep deprivation has been shown to change the body’s basic metabolism and the balance between fat and muscle mass.

A review of existing studies found permanent night-shift workers were 29 % more likely to become overweight. They were also 41 % more at risk of a heart attack or stroke.