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

Help Link : Café et vieillissement perturbent le sommeil lent profond Link : Napless cats awaken interest in adenosine Link : Adenosine receptors and behavioral actions of methylxanthines
Original modules
Tool Module : Cybernetics Cybernetics

Knowing that the body releases adenosine continuously when a person is awake, some researchers claim that caffeine would be more effective if people took it in small doses throughout the day, rather than in large amounts at a particular time of day (for example, by drinking several cups of coffee at breakfast).

Link : La caféine serait mal utilisée?


Research indicates that for someone to be able to fall asleep, two bodily processes must be properly synchronized. The first is the circadian rhythm, which has a 24-hour period and is governed by your body’s biological clock. The circadian rhythm controls the cyclical secretion of several hormones, including melatonin, that are involved in sleep. The second process is the accumulation of hypnogenic substances in your body for 16 hours every day. These substances induce a desire to sleep that does not go away until you in fact get some sleep.

Thus you can fall asleep only when two conditions have been met: your body’s biological clock must have brought it into a hormonal balance conducive to sleep, and it must have been a good while since you last slept, so that your levels of hypnogenic substances have built up sufficiently.

The hypnogenic substance that has been the subject of the most research is adenosine, a small molecule that acts as a neuromodulator in a great many synapses in the brain. In order to stay awake longer, people have long consumed naturally occurring antagonists to the brain’s adenosine receptors. Both the caffeine in coffee and the theophylline in tea are examples of such adenosine antagonists and are well known for their stimulant effects.

Diagram of an adenosine molecule
 (Atoms: green = carbon, blue = nitrogen,
red = oxygen, white = hydrogen,
pink = phosphorus)

It was in the early 1980s that scientists first discovered the chemical mechanism by which drinking coffee helps people to stay awake: caffeine, the psychoactive substance in coffee, prevents adenosine from binding to certain neurons in the brain.

Once this discovery was made, adenosine became a subject of interest for more and more neurobiologists who were doing sleep research. Numerous animal experiments eventually confirmed that adenosine definitely plays a role in the sleep/wake cycle. Some of the experimental findings that led to this conclusion: a) blocking the effects of adenosine made animals more alert; b) injecting animals with an adenosine agonist caused them to fall asleep; c) in certain parts of the brain, the concentration of adenosine normally increases naturally during the day and decreases at night, but if animals are forced to stay awake at night, this concentration keeps increasing.

These experiments thus showed that adenosine, along with other chemicals such as serotonin and melatonin, is one of the molecules whose concentration in the brain influences the onset of sleep.

But how exactly does adenosine exert this influence? During periods of wakefulness, neuronal activity increases the concentration of adenosine, which has an inhibitory effect on a great many neurons. Among these are the neurons of the hormonal systems that are the most active when we are awake: the norepinephrine, acetylcholine, and serotonin systems. Experiments have shown, for example, that when the levels of adenosine in the basal telencephalon are raised artificially, the neurons in this structure that project axons throughout the cortex produce less acetylcholine. As a result, cortical activity slows, and the individual falls asleep.

The synchronized brain activity characteristic of non-REM sleep can then become established. But once non-REM sleep has continued for a while, the adenosine levels begin to decline. The systems responsible for wakefulness can then start becoming more active, causing the individual to awaken and the cycle to begin all over again. Thus we see that the sleep/wake cycle involves a highly efficient negative feedback loop (for more on feedback loops and cybernetics, follow the Tool module link to the left).

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