We spend about one-third of our lives sleeping. A lot of things are happening to us when we sleep. Body functions slow down. Heart rate is less. Breathing slows down. The amount of acid in the stomach decreases. The kidneys continue to filter waste, but at a lesser rate.

There are five stages of sleep. The first four are regarded as quiet phases. Blood flow to the brain is much less. Blood is diverted to the muscles. Our brain cycles through the stages in approximately 90-minute intervals. The last stage is REM (rapid eye movement).

During the REM stage, there is a high level of brain activity. The eyes move back and forth and up and down, as if we are trying to focus on something. During REM, parts of the brain we use for learning become very active. It is during the REM stage that we dream.

Signals are sent to the motor neurons in the spinal cord. The muscles in the arms and legs become non-active, much like being paralyzed, so that we don’t act on those dreams.

During REM, blood flow to the brain increases to areas we use for memory and emotional experiences.  Blood flow decreases to areas of the brain that are used for complex reasoning and language.

Sleep is a time for healing and for our body to do repairs. People who are don’t get enough sleep have difficulty thinking quickly, remembering details, and have trouble with math problems. Poor sleep patterns are linked to poor health and lower life expectancy. Sleep helps fight infectious diseases. There is evidence that lack of sleep reduces levels of white blood cells. White blood cells help fight infection.

When we are sleeping, a cancer killing protein molecule TNF (tumor necrosis factor) is pumped through our blood stream. If a person is sleep-deprived, those levels are down considerably.

When we are awake, the body used oxygen and food to provide energy. More energy is spent than conserved.  Energy is conserved during sleep, and repair and growth take over. Adrenaline levels drop, but the body produces the protein human growth hormone (HGH).  All the tissues and bones of the body are undergoing growth, maintenance, and repair.

We humans have a built-in body clock called the circadian rhythm. It causes 24 hour changes in many body activities, and governs the daily alternation between sleep and waking. It lets the body know when sleep is coming. The term circadian means “about a day”.

The circadian rhythm regulates all the processes of the body. A network of chemical messengers and nerves are controlled by the circadian clock. Getting regular periods of sleep at night helps the body clock regulate hormone production.

Body temperature falls during the night. By morning, body temperature is about two degrees Fahrenheit lower than in early evening. A person changes sleep position several dozen times during the night. But the muscles remain relaxed.

Sleep apnea is a disorder in which a person has abnormal pauses in breathing. These pauses can last from a few seconds to minutes. Sleep apnea is diagnosed with an overnight sleep test.

Sources:  www.sleepfoundation.org  and  http://www.howstuffwork.com