What is sleep and why do we do it? People have pondered these questions since the days of Plato and Aristotle, and today their contemporary counterparts are still looking for answers. Sleep medicine didn’t become mainstream until the 1980’s when physicians began recognizing the signs and symptoms of sleep disorders. In the 1990’s, federal agencies became concerned about sleepy drivers, pilots and workers in facilities such as nuclear power plants.

The early theories about sleep sound quite absurd, but their basis transcends time and has relevance in today’s modern sleep research community.

Dust off the Ancient History Books

The great thinkers of the 4th and 5th centuries BC correlated body “humors” (blood and bodily fluids) with sleep generation. They considered sleep to be a short ‘death’, as opposed to a long death, which is permanent. Consider these theories on sleep:

• Alcmaeon of Greece (6th century BC) proposed that sleep was caused by blood filling the vessels in the brain. When the brain vessels emptied, the person woke up.
  
  
• Empedocles of Sicily (5th century BC) believed sleep occurred when the blood cooled. He theorized that sleep occurred when fire separated from the other elements of earth, air and water, thereby cooling the blood and causing sleep.
  
  
• Hypocrites (4th century BC) took the opposite approach of Empedocles. He said that sleep resulted from the warming of blood as it passed from the limbs to the core of the body.
  
  
• Plato and Aristotle (4th century BC) theorized that vapors from food decomposition rose into the brain to cause sleep. This was widely accepted until the 15th or 16th century. Interestingly, people were aware of the influence of alcohol on sleep since biblical times, so perhaps people assumed that “vapors” similar to alcohol’s promoted sleep.

Sleep Moves with the Times

Most physicians in the early 18th century—and for many centuries afterward--believed sleep was a passive process. They said that the brain simply reacted to things or processes being “done to it.” For example, in the 18th century, researchers said that sleep occurred when blood flowed into the head and pressured the brain, similar to what Alcmaeon proposed in the 6th century BC.

Things changed in the 19th century, when Leonard Hill proposed that intracranial pressure and blood flow was unchanged during sleep. The medical community also felt that the brain was completely inactive during sleep. In effect, they theorized that without stimulation the brain shut down and went to sleep. Like their ancient Greek counterparts, these modern physicians believed sleep was still a state between wake and death. Darkness was identified as a contributing factor toward brain passivity that helped the brain shut down so it could sleep.

Sleep Comes of Age

At the turn of the century, a popular theory speculated that sleep was the result of a build up of toxins in the body. When sleep occurred, the toxins went away. Since alcohol was known to promote sleep, opium was also included as a sleep promoter. During this time, the medical community also realized that caffeine, in the form of coffee, was a sleep inhibitor.

The toxin theory became widely popular when two French physiologists, Legendre and Pieron, conducted experiments showing that when blood serum taken from sleep deprived dogs was injected into normal dogs, sleep was promoted in these normal dogs.

In the 1920’s, Nathaniel Kleitman, a physiologist at the University of Chicago, observed that subjects who stayed up all night had difficulty staying awake in the middle of the night, but found it easier to stay awake during the daylight hours. This contradicted the toxin theory, which stated that the build up of toxins forced the brain to sleep.

The generally accepted image of the passive sleeping brain was finally put to rest in the 1930’s when Hans Berger of Germany began using surface electrodes placed on the subject’s head to record electrical activity from the brain). He also identified different electrical signals, or brain waves as we call them today. Berger coined the term EEG, which stands for electroencephalogram.

Kleitman is also credited with noticing the strange eye movements exhibited in sleeping people, but it wasn’t until the 1950’s that he and other researchers used the EEG electrodes on sleeping patients to identify distinct waveforms and the rapid eye movements (REM) that he had observed in children and adults.

By the end of the 1950’s, researchers were able to classify sleep into two very different stages, non-REM and REM. REM sleep was associated with rapid eye movements and muscle atonia (muscles are inactive). Non-REM had many different types of brain wave forms and muscles of the body were active, but not necessarily moving. Content of dreams was associated with the REM part of sleep.

So . . . Do We Know Why We Sleep?

While the exact physiological mechanisms of why we sleep are still unknown, great progress has been made. It appears that sleep is a combination of hormones, brain chemistry, brain pharmacology, body temperature and a host of other systems.

William Dement, in his book, The Promise Of Sleep, writes, “sleep is a wall between the conscious mind and the outside world.” He also offers that sleep is a “reversible” process, unlike coma, hibernation and anesthesia where the sleeper cannot awaken. Sleep is also naturally occurring and periodic.

We do know that sleep has restorative properties. If we have a good night’s sleep, we feel good the next day and can perform our daily tasks. Brain tissue is also restored as a result of sleep. Sleep allows us to conserve energy to perform the physical and mental tasks that challenge us every day.

We know more about the mechanics of sleep than the ancient Greeks, but sleep still offers the medical community an elusive, mysterious field of study. Bring on the international symposiums, the famed research projects and the clinical studies. Regardless of new advances in research, sleep will remain a phenomenon of the human experience.

For more information on sleep and sleep disorders, check out the National Sleep Foundation's Web Site at www.sleepfoundation.org