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

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The Various Speeds at Which We Perceive Time

One of chronobiology’s important contributions has been to demonstrate that the human body reacts differently to medications according to the time of day when they are administered. This idea was scarcely recognized in the early 1980s. Now it has become the foundation of an entire discipline, known as chronopharmacology.

By taking the human body’s internal circadian rhythms into account, health care professionals can recommend a time of day when taking a given medication will optimize its benefits or, in some cases, reduce its side effects and/or toxicity. For example, some medications that act on certain hormones have no effect at all if taken at 6:00 PM but are fully effective if taken at 7:00 AM.

Link : la chronopharmacologie vient d'obtenir sa reconnaissance scientifique Link : Livre : Chronopharmacologie

One noteworthy characteristic of the human biological clock is that it is independent of ambient temperature —one of the rare systems in the human body that is not slowed down when the ambient temperature is cold or sped up when it is hot. This ability of the body’s molecular clockwork to compensate for temperature is essential, because it must maintain its circadian rhythm in both summer and winter.


The behaviour of almost all land animals, including humans, follows rhythms that are of endogenous origin but that are also modulated by the daily variations in light and darkness. These cyclical fluctuations in behaviour are known as circadian rhythms.

Circadian rhythms are biochemical, physiological, and behavioural cycles whose period is approximately 24 hours. These cycles are co-ordinated by molecular oscillators in the neurons of the suprachiasmatic nucleus. These oscillators represent the key component in the human biological clock, which is synchronized with the alternation of day and night by specialized light-sensitive cells in the retina.

The reason that the human biological clock needs to be continuously adjusted to the level of the ambient light is that its endogenous cycle does not last exactly 24 hours. The actual length of this period has been studied in numerous experiments with complete temporal isolation, in which the subjects were deprived of any light or auditory cues that might indicate the time of day. The values that these studies have found for the period of the natural human circadian cycle ranged from 24.2 to 25.5 hours. Thus the Latin roots of the word “circadian”—circa, meaning “around” and dies, meaning “a day”—are quite apt.

It is this light-entrained adjustment mechanism that lets the body’s central clock track the alternation of day and night precisely. This central clock in turn co-ordinates the activity of many other biological clocks that are located in various peripheral tissues and that have their own molecular oscillators. This is why most of the body’s major physiological functions fluctuate with the time of day. Examples include body temperature, hormone secretion, urine production, blood circulation, metabolism, and even the growth of hair!

These fluctuations usually go through a peak and a trough that coincide with particular times of day. For example, human body temperature is always lowest at night.

Adapted from: Gerry Wyder

Of course, body temperature can also fluctuate under exogenous influences such as physical activity levels, stress levels, the presence of infection, or simply the ambient temperature. But in experiments where subjects lie awake but still for 30 hours or more, endogenous variations in their body temperature are also observed. In addition to the major drop in body temperature at night, there is also a slight decline from early to mid-afternoon. This latter temperature drop, far more than having a full stomach after lunch, would appear to explain the reduced alertness and sleepiness that many of us experience at this time of day.


Other physiological parameters also undergo substantial endogenous fluctuations over the course of the day. The secretion of several different hormones is one example. During the daytime, the levels of melatonin, a hormone manufactured in the pineal gland, are almost undetectable in the blood. The pineal gland starts to secrete melatonin in mid-evening, as darkness falls, and its secretion peaks between 2:00 and 4:00 in the morning.

In the case of the hormone cortisol, peak secretion occurs just before a person wakes up, so that this hormone’s level is highest when the person gets out of bed, thus contributing to the general activation of the body.

The secretion of human growth hormone, which is essential for bone and muscle growth in children, takes place mainly during deep sleep, which occurs mostly at the start of the night. In adults, this hormone plays an important role in metabolism, promoting the synthesis of proteins, helping to burn fats, strengthening bones, and so on.

Wakefulness and sleepiness are the two aspects of a single fluctuating state whose circadian cycle is divided into two sub-cycles of about 12 hours each. In other words, people who are placed in an environment devoid of time cues will display a bidaily rhythm of propensity to sleep.

The first and longest period of sleepiness occurs around the time that you are used to going to bed and is deepest between 3:00 AM and 6:00 AM. This is the time of day when your metabolism and body temperature are at their lowest. So is your alertness; if you are awake, you are physically clumsier, and your mind feels sluggish.

The second daily period of sleepiness occurs 12 hours later, between 2:00 PM and 4:00 PM. This period is shorter than the one that occurs at night, but we all know it well—it’s the mid-afternoon slump. Contrary to popular belief, it has nothing to do with the heat of the afternoon or with digesting the noontime meal. Studies have shown that people who live in the warm temperatures at the Equator experience two troughs in their wakefulness/sleepiness cycle just as North Americans do, and that people feel sleepy in the afternoon even if they haven’t eaten any lunch. (Moreover, most people don’t experience this same kind of sleepiness after breakfast or dinner.)

Source: Dr. Guilhem Pérémarty

Thus, the fluctuations in our wakefulness do in fact depend on our internal biological clocks. And a short nap in the afternoon would appear to be beneficial for most people.

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