Have you ever wondered what is sleep and why do we need it? One third of our life is spent sleeping! Twenty five percent of that time our eyes are moving as we experience vivid, emotional dreams.

REM sleep

These periods of rapid eye movement are commonly known as REM sleep. All terrestrial mammals appear to exhibit REM sleep, which alternates with non-REM sleep (quiet sleep) in a regular cycle. Brain activity during REM resembles that of the waking brain, it is buzzing. Yet, in a human, the body is essentially paralyzed. However, dolphins and other marine mammals swim while sleeping and some birds may sleep while flying during long migrations. In humans, seven hours of sleep a night correlates with the longest life spans. Yet there is a huge variation in the amount of sleep that different species of animals need. For example, an elephant needs only three or four hours of sleep per day, a cat about 13 to 16 hours, while an opossum sleeps for 20 hours a day. Size appears to be the major determinant: the bigger the animal the less sleep they need. Smaller animals have higher metabolic rates and higher brain and body temperatures than large animals.

Purposes of REM and non-REM sleep

What is non-REM sleep for? There exists a very small group of brain cells, about 100,000, at the base of the forebrain that are maximally active during non-REM sleep and appear to be responsible for inducing sleep. The rest of the brain is like an idling automobile using little energy. Vivid dreams are rare in non-REM sleep. Non-REM dreams consist of brief, fragmentary impressions, that are less emotional than REM sleep. In the body, metabolism generates free radicals that are known to damage and even kill cells. Thus, the hypothesis is that reduced activity during non-REM sleep may give many brain cells a chance to repair themselves.

REM sleep appears to serve multiple purposes. Since brain activity during REM sleep resembles that which occurs during waking, energy consumption is just as high. However, the brain stops releasing the neurotransmitters that activate the brain cells controlling muscles, except for those that move the eyes. Scientists are now able to watch the sleeping brain at work. The areas of the brain that generate internal imagery are active even though the regions that receive signals from the eyes are shut down. Regions responsible for short-term memory become inactive and areas involved in judgment wind down. Hence we tend to forget our dreams and accept the rapid illogical shifts of scenes. Using PET and fMRI technology scientists have found that during dreaming one of the most active brain areas is the limbic system. The limbic system is found deep in the middle of the brain and functions as an emotional-processing network.

Limbic system interweaves conscious and unconscious

The primary structures of the limbic system are all primitive organs that have been conserved throughout mammalian evolution. The human cognitive processes appear to involve circuits that are independent of these emotional circuits. The limbic system interweaves these unconscious primal emotions with our conscious cognitive thoughts and perceptions. This produces our complex repertoire of human emotions -- envy, delight, surprise, etc. In fact, dreams may reflect a fundamental aspect of the mammalian memory processing. Memory is part of the brain's attempt to impose order on the environment. Information acquired during waking states is reprocessed and consolidated during sleep. Hence, one function of the limbic system, particularly during REM sleep, is to tie together emotions and memory.