Stages of Sleep

The old myth that sleep is a rest period for the brain is definitely not correct. In fact, the brain (1) is extremely active during sleep.

First of all, the brain is what tells us when it is time to sleep. Sensation of light taken in from the retina is transferred through the nerves to a minute structure of the brain called the hypothalamus. This hypothalamus determines the amount of light exposure and adjusts the body accordingly. This is all part of the circadian rhythms and the body's sleep-wake cycle.

People aren't certain as to the exact function of sleep, but some speculate that during certain stages of sleep, the brain is testing, strengthening, or somehow improving our neurons.There are companies that would have you believe that you can play a learning tape, such as one for a foreign language, in your sleep, and learn the subject without any effort because your brain can do your work for you while you sleep.

But is it really possible for people to learn in their sleep?

To measure the voltage of the electrical impulses being given of by the brain's neurons, technicians can use an EEG machine. The type of brain wave, defined by amplitude and frequency, helps determine the type of sleep that the sleeper is experiencing. The state of falling asleep is called the hypnagogic state.

The brain follows cycles during sleep. These cycles last between 90 and 100 minutes each, therefore repeating 4 or 5 times in a course of one night. Sleep is categorized into two different types: NonREM sleep and REM sleep. The cycle begins with NonREM sleep: stages one, two, three, then four. Then, the stages quickly reverse, reaching REM sleep, which is the dreaming period. The cycle then starts over. As the cycles continue during the night, there is less deep sleep (stages 3 and 4) and more dreaming (REM sleep). The proportional amount of sleep we get also changes with age.

Here is a chart showing a little about the brain waves during different states & stages (2):

The neurotransmitters serotonin and norepinephrine are both most active during NonREM sleep. The pons (1) activate the neurotransmitter acetylcholine only during REM sleep and while one is awake.

In REM sleep the pons is activated, exciting the medullary inhibitory area by projections (tegmento-reticular tract) which connects the pons to the inhibitory center.

The medullary center inhibits the motor neurons and gives rise to atonia. A lateral locomotor strip, down the outside of the brain stem, plays an important role in the reduction of motor drive. It is connected to structures in the spinal cord. In REM sleep the pons stimulates the inhibitory zone, turning off the locomotor strip and shutting down motor drive.

(1) Images of brain. Permission granted for use of images and for links to her site by Silvia Helena Cardoso, PhD,Center for Biomedical Informatics State University of Campinas, Brazil by email message, Aug. 1999. She is the editor of Brain and mind: electronic magazine on neuroscience. http://www.epub.org.br/cm/n02/mente/neuroestrut_i. htm and http://www.epub.org.br/cm/n02/mente/neurobiologia_i.htm are about the neurobiology of dreams. http://www.epub.org.br/cm/n02/mente/desligamento_i.htm is about REM sleep disorder, and why we don't usually act out dreams.

(2) This chart (Original Graphic by Gerry Wyder). was compiled based on information from the following sources:

Hobson, J. Allan. Sleep. New York: Scientific American Library, 1989 p. 15
Ancoli-Israel, Sonia. All I Want is a Good Night's Sleep. St. Louis: Mosby, 1996. p. 6-10