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Sleep and dreams
The Sleep/ Dream/ Wake Cycle
Our Biological Clocks

Help Link : Lecture 1: Biology in Four Dimensions, by Joseph S. Takahashi, Ph.D. Link : Clocks and Rhythms Link : Pinéal
Link : LA GLANDE PINÉALE OU EPIPHYSE Link : Pineal gland
Research : Shimon Amir
Original modules
Tool Module: Your “Mental Stopwatch” Your “Mental Stopwatch”

A number of studies have shown that the central oscillator in the SCNs is not the only biological clock in the human body, and that the genes of the human biological clock are also expressed cyclically in many peripheral tissues, such as the liver, heart, skin, and lymphocytes.
These peripheral clocks are co-ordinated by the central clock in the SCNs, which is synchronized with the day/night cycle by the direct influence of the ambient light. This central clock then communicates with the peripheral ones via the nervous and circulatory systems, so that these peripheral clocks can in turn synchronize themselves with the central oscillator. These peripheral clocks may also be influenced by other entraining factors, such as the timing of meals.

Link : Peripheral Timekeeping: Mammalian Cells Outside The Brain Have Their Own Circadian Clocks Link : Circadian clock genes oscillate in human peripheral blood mononuclear cells Link : Vous vous endormirez plus savant ce soir...
Link : Le gène circadien double time code pour une kinase Link : The circadian cycle: daily rhythms from behaviour to genes Link : Les noyaux suprachiasmatiques : une horloge circadienne composée

In addition to affecting the production of melatonin indirectly, via the pineal gland, the suprachiasmatic nuclei may also release certain peptides directly into the cerebrospinal fluid, such as arginine vasopressin (AVP) during the daytime and vasoactive intestinal peptide (VIP) at night.

Link : Annual variations in the vasopressin neuron population of the human suprachiasmatic nucleus Link : Dynamic changes in the immunoreactivity of neuropeptide systems of the suprachiasmatic nuclei in golden hamsters during the sleep-wake cycle


There is a part of the brain that can maintain a basic, independent rhythm even if the external cues of the cycle of day and night are eliminated.

Scientists have determined that in mammals, this “biological clock” is located in two tiny structures called the suprachiasmatic nuclei (SCNs), located in the left and the right hypothalamus and bordering on the third ventricle. The neurons of these two nuclei are among the smallest in the brain.

To maintain its accuracy, this central biological clock resynchronizes itself each day with external stimuli such as the brightness of the ambient light, by means of the optic nerves, which bring this information to it from special ganglion cells in the retina.

The output pathways from each SCN consist of axons that innervate mainly the hypothalamus and nearby structures. Some of these axons also project to other parts of the diencephalon (forebrain), while others project to the mesencephalon (midbrain).


Lastly, both SCNs also send signals, though indirectly, to another very important structure: the pineal gland. In birds, reptiles, and fish, this small gland located at the top of the brain is sensitive to light and co-ordinates some cyclical phenomena on its own. In mammals, however, though the pineal gland does retain its ability to produce secretions cyclically (specifically, the hormone melatonin, at night), it does not constitute a clock on its own; instead, its cyclical synthesis of melatonin is controlled by timing signals that it receives from the SCNs.

Each day, the pineal gland begins to produce melatonin (sometimes called the “sleep hormone”) as night falls. As the level of melatonin in the blood rises, body temperature falls slightly, and the individual feels sleepier and sleepier. The melatonin level remains high for just about 12 hours, then starts falling again in the early morning, as daylight inhibits this gland’s activity.

Scientists do not yet know the details of how the central biological clock in the SCNs regulates so many different human cyclical behaviours.

But scientists do know that it uses the pineal gland to do so, and they have shown that destroying the SCNs’ output pathways also destroys the body’s circadian rhythms.

Because GABA is the essential neurotransmitter for almost all of the SCNs’ neurons, one would expect an inhibitory effect on the neurons that they innervate. In addition to sending out messages along these axonal pathways, the SCNs’ neurons seem to secrete a neuropeptide called vasopressin in a cyclical pattern. It has been shown that this pattern arises because the segment of the gene for vasopressin that causes it to be produced (its “ON” switch, so to speak) is controlled by the same proteins as the genetic feedback loops that are the molecular basis for the human biological clock. Lastly, it should be noted that the effects of the vasopressin that is secreted by the neurons of the SCNs are limited to the brain, contrary to those of pituitary vasopressin, which is involved in the water metabolism of the entire body.

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