The yin-yang symbol from ancient China demonstrates many dualities. Called the Taijitu, this familiar symbol shows two different but related and dependent forces or states. Yin and yang are complementary and the sign shows how they mix and are in close contact, hugging each other.
Sleep and waking are not separate and distinct states, even though we think of them that way in a shorthand description of human behavior. Is it a coincidence that the American Academy of Sleep Medicine uses a variation of the yin-yang symbol as its logo?
The small circle of white inside the black and the small circle of black inside the white. These find analogies in sleep, too. Everyone briefly wakes during the night, even people who feel they are sleeping through the night. Microsleep or short naps can punctuate waking, and there is evidence that at some level, portions of the brain can fall asleep while most of the brain remains awake.
The mechanism for transition between waking and sleep is of interest. Indeed, the class of sleep disorders called parasomnias is about the transition between states, but even without consideration of disorders, neuroscientists want to know how the brain falls asleep and wakes up.
As measured by EEG readings, the transition from the fast waves characteristic of waking to the slow waves characteristic of sleeping can take anywhere from a few seconds to several minutes,
The brain has systems to promote sleep and to promote waking. These physiological systems are complex and it is overly simplistic to think of them just battling it out in the brain. The brain is designed like a flip-flop switch so that most of the time we can be said to be asleep or awake, even if part of the brain are in a minority and differ from the organism as a whole.