Sleep works as a restorative function for the brain and body.  Deprive a person of proper sleep & physical symptoms invariably develop.  The mechanism & function of sleep and it's relationship to health and disease is a field of health that is being increasingly emphasized.

True rest occurs in deep sleep

University of Chicago Researchers found that the first stage of age-related deterioration of sleep quality in men occurs between the ages of 25 and 45.

• They're sleeping as long as younger men, but they're spending less time in restorative deep sleep, the scientists write in the Journal of the American Medical Association.

• The scientists studied 149 healthy men ages 16 to 83.

• They found that the proportion of deep sleep drops from nearly 20% of a normal night's sleep for men under 25 to less than 5% for those over 35.

• By age 45, few men spend much time in deep sleep.

• Sleeping less deeply doesn't just mean that thirty something men might be more likely to be awakened by street noise outside the window. (This study did not find that more frequent awakenings during the night accompanied declines in deep sleep.) 

• It also means that their bodies are secreting less growth hormone, the study found. In recent years, studies in the elderly have linked growth hormone deficiency to increased obesity, loss of muscle mass and reduced exercise capacity.

There are two cycles involved in our sleep process.  Circadian rhythms regulate the 24 hour daily interval & healthy sleep is comprised of 90 minute sleep cycles.

There are 5 stages of sleep that cycle every 90 minutes or so throughout the night.  Proper restful sleep will cycle through these 5 stages of sleep every 90 minutes throughout the night.