The SCN takes the information on the lengths of the day and night from the retina, interprets it, and passes it on to the pineal gland, a tiny structure shaped like a pine cone and located on the epithalamus. In response, the pineal secretes the hormone melatonin. Secretion of melatonin peaks at night and ebbs during the day and its presence provides information about night-length.
Several studies have indicated that pineal melatonin feeds back on SCN rhythmicity to modulate circadian patterns of activity and other processes. However, the nature and system-level significance of this feedback are unknown.
A great deal of research on biological clocks was done in the latter half of the 20th century. It is now known that the molecular circadian clock can function within a single cell; i.e., it is cell-autonomous.[13] At the same time, different cells may communicate with each other resulting in a synchronised output of electrical signaling. These may interface with endocrine glands of the brain to result in periodic release of hormones. The receptors for these hormones may be located far across the body and synchronise the peripheral clocks of various organs. Thus, the information of the time of the day as relayed by the eyes travels to the clock in the brain, and, through that, clocks in the rest of the body may be synchronised.
Circadian rhythmicity is present in the sleeping and feeding patterns of animals, including human beings. There are also clear patterns of core body temperature, brain wave activity, hormone production, cell regeneration and other biological activities.
A number of other disorders, for example bipolar disorder and some sleep disorders, are associated with irregular or pathological functioning of circadian rhythms. Recent research suggests that circadian rhythm disturbances found in bipolar disorder are positively influenced by lithium's effect on clock genes.[55]
http://en.wikipedia.org/wiki/Entrainment_%28physics%29
Entrainment is the process whereby two interacting oscillating systems, which have different periods when they function independently, assume the same period. The two oscillators may fall into synchrony, but other phase relationships are also possible.
The system with the greater frequency slows down, and the other accelerates. Christian Huygens, a notable physicist, coined the term entrainment after he noticed, in 1666, that two pendulum clocks had moved into the same swinging rhythm, and subsequent experiments duplicated this process. Notably, the two pendula stabilized not in synchrony, but in antiphase. They satisfy the definition of entrainment because they have the same period, even though they have opposite phase. The accepted explanation for this is that small amounts of energy are transferred between the two systems when they are out of phase in such a way as to produce negative feedback. As they assume a more stable phase relationship, the amounts of energy gradually reduce to zero. In the realm of physics, entrainment appears to be related to resonance.
http://en.wikipedia.org/wiki/Synchronization_of_chaos
Synchronization of chaos is a phenomenon that may occur when two, or more, chaotic oscillators are coupled, or when a chaotic oscillator drives another chaotic oscillator. Because of the butterfly effect, which causes the exponential divergence of the trajectories of two identical chaotic system started with nearly the same initial conditions, having two chaotic system evolving in synchrony might appear quite surprising. However, synchronization of coupled or driven chaotic oscillators is a phenomenon well established experimentally and reasonably well understood theoretically.
It has been found that chaos synchronization is quite a rich phenomenon that may present a variety of forms. When two chaotic oscillators are considered, these include:
Identical synchronization. This is a straightforward form of synchronization that may occur when two identical chaotic oscillators are mutually coupled, or when one of them drives the other.
Generalized synchronization. This type of synchronization occurs mainly when the coupled chaotic oscillators are different, although it has also been reported between identical oscillators.
Phase synchronization. This form of synchronization, which occurs when the oscillators coupled are not identical, is partial in the sense that, in the synchronized state, the amplitudes of the oscillator remain unsynchronized, and only their phases evolve in synchrony.
Anticipated and lag synchronization. In these cases the synchronized state is characterized by a time interval...
Amplitude envelope synchronization. This is a mild form of synchronization that may appear between two weakly coupled chaotic oscillators. In this case, there is no correlation between phases nor amplitudes; instead, the oscillations of the two systems develop a periodic envelope that has the same frequency in the two systems. This has the same order of magnitude than the difference between the average frequencies of oscillation of the two chaotic oscillator. Often, amplitude envelope synchronization precedes phase synchronization in the sense that when the strength of the coupling between two amplitude envelope synchronized oscillators is increased, phase synchronization develops.
All these forms of synchronization share the property of asymptotic stability. This means that once the synchronized state has been reached, the effect of a small perturbation that destroys synchronization is rapidly damped, and synchronization is recovered again. Mathematically, asymptotic stability is characterized by a positive Lyapunov exponent of the system composed of the two oscillators, which becomes negative when chaotic synchronization is achieved.
Some chaotic systems allow even stronger control of chaos. Both synchronization of chaos and control of chaos constitute parts of Cybernetical Physics.
In chaos theory, control of chaos is based on the fact that any chaotic attractor contains an infinite number of unstable periodic orbits. Chaotic dynamics then consists of a motion where the system state moves in the neighborhood of one of these orbits for a while, then falls close to a different unstable periodic orbit where it remains for a limited time, and so forth. This results in a complicated and unpredictable wandering over longer periods of time.
Control of chaos is the stabilization, by means of small system perturbations, of one of these unstable periodic orbits. The result is to render an otherwise chaotic motion more stable and predictable, which is often an advantage. The perturbation must be tiny, to avoid significant modification of the system's natural dynamics.
http://en.wikipedia.org/wiki/Cybernetical_physics
Until recently no creative interaction of physics and control theory (cybernetics) has been seen and no control theory methods have been directly used for discovering new physical effects and phenomena. The situation has dramatically changed in the 1990s when two new areas emerged: control of chaos and quantum control.
In 1990 the paper [1] was published in Physical Review Letters by Edward Ott, Celso Grebogi and James Yorke from the University of Maryland discovered that even small feedback action can dramatically change behavior of a nonlinear system, e.g. turn chaotic motions into periodic ones and vice versa.
http://en.wikipedia.org/wiki/Cybernetics
Cybernetics is the interdisciplinary study of the structure of regulatory systems. Cybernetics is closely related to control theory and systems theory. Both in its origins and in its evolution in the second-half of the 20th century, cybernetics is equally applicable to physical and social (that is, language-based) systems.
Cybernetics is most applicable when the system being analysed is involved in a closed signal loop; that is, where action by the system causes some change in its environment and that change is fed to the system via information(feedback) that causes the system to adapt to these new conditions: the system's changes affect its behavior. This "circular causal" relationship is necessary and sufficient for a cybernetic perspective.
http://afp.google.com/article/ALeqM5jyOAlQsekPlvQmluyXlU1gL60iQw
Human evolution speeding up: study Dec 10, 2007 CHICAGO (AFP) — The world may feel more and more like a global village, but its residents are increasingly genetically diverse thanks to the rapidly accelerating pace of human evolution, a study said Monday.
Geneticists say the huge explosion in our numbers in the past 40,000 years, since Homo sapiens migrated out of Africa to other continents, has resulted in a much faster pace of evolution compared to the previous six million years.The pace of change has increased 100-fold in modern times compared to our distant past, and most notably since the Ice Age, 10,000 years ago, and has led to increasing diversification between the races.
"We are more different genetically from people living 5,000 years ago than they were different from Neanderthals," said John Hawks, an anthropologist at the University of Wisconsin-Madison who collaborated on the study.
The findings are based on analysis of data from an international genomics project. A team of scientists examined DNA from 270 individuals in four ethnically different populations to see how genetic variations or SNPs (single nucleotide polymorphisms) evolved over time.
Contrary to conventional wisdom, which holds that human evolution has slowed to a crawl or even stopped in modern humans, the researchers' analysis suggested that the process of natural selection has sped up.
"Rapid population growth has been coupled with vast changes in cultures and ecology, creating new opportunities for adaptation," the authors wrote in the paper in the Proceedings of the National Academy of Sciences.
"Human races are evolving away from each other," said Henry Harpending, an anthropology professor at the University of Utah in Salt Lake City.
"Genes are evolving fast in Europe, Asia and Africa, but almost all of these are unique to their continent of origin. We are getting less alike, not merging into a single, mixed humanity."
He said that is happening because humans dispersed from Africa to other regions 40,000 years ago and "there has not been much flow of genes between the regions since then."
