Clocks and Rhythms
By adaptation in time we should understand phenomena occurring both in the relationships between the organism and its environment and in "the internal economy" of the organism, as in the secretion of a hormone or regulation of membrane permeability.
Up to mid-20th century we believed those temporal adjustements to be carried out by machanisms of action-reaction type, as for example in the case of our sleep/wake cycle: darkness would promote sleep and light, wakefulness.
The demonstrations of the persistence of that cycle in temporal isolation (people living in caves for weeks or months keep sleeping and waking with a periodicity of around 25h) in several species suggested the existence of "biological clocks", that is, mechanisms able to generate cycles independently of environmental signs. CLOCKS are thus mechanisms generationg cycles and their end product are the observable biological RHYTHMS. The discipline dealing with those questions is known as CHRONOBIOLOGY.
Fig.1. Daily rhythms depend on the suprachiasmatic nucleus receiving input about day lenght (light levels) from the eyes and about external temperature from the skin. This nucleus then integrates the body's 24-hour rythms, which include hormone levels, temperature, general alertness and urine output.
|Today the existence of "biological clocks" is widely accepted by the scientific community and some of these structures have been identified in several species, as groups of neurons able to generate cycles. The suprachiasmatic nuclei of the anterior hypothalamus in mammals are the best known exemple of clocks that regulate circadian (about 24h) rhythmicity (fig.1) . It is now also known the genetic determination of these temporal controls (1)|
An updated review of the phenomena linked to biological rhythmicity may be found (in Portuguese) in our book "Cronobiologia: Princípios e Aplicações" (2). This book has also been published in Spanish by the EUDEBA (Editora de la Universidade de Buenos Aires). Updated information on Chronobiology (details of our book, for instance) may be obtained by visiting our Homepage Grupo Multidisciplinar de Desenvolvimento e Ritmos Biológicos, where you will be able to visit virtually and contact via e-mail several labs in Brazil and abroad.
An example of a biological rhythm: core temperature
Core temperature is the temperature of arterial blood in the central regions of the body. Reliable measurements of core temperature may be obtained with catheters with thermistors inserted in arteries such as the carotida, femural, aorta and other arteries.
Non-invasive alternatives are: oral, rectal and tympanic temperature. Skin temperatures, like the axilar, show peripheral thermoregulation adjustements which do not necessarily coincide with core temperature values.
Check the result of repeated measurements of the core temperature of a healthy young adult along 24 hours. The curve represents an ideal situation in which the temperature data were collected regularly every 60 minutes both during sleep and wakefulness.
Notice the decline in core temperature along the night and the nadir (through) around 5 am. Just after that the temperature starts rising. That is when he will probably wake up – see the graph.
Around 11 pm his temperature is declining and he feels sleepy.
Around noon his temperature declines a little and he feels "heavy" which may send him to a nice nap. Some researchers call this moment "secondary sleep" or "secondary sleep gate".
If he decides to have a big meal for lunch and lie down right after, his temperature will show a further declination, in what I am calling the "feijoada effect" (feijoada is a tipical brazilian dish, spicy and heavy); this phenomenom has also been called "post lunch dip".
Now suppose he was engaged in heavy physical exercise early in the morning. See the "exercise effect" in the graph.
These changes produced by the heavy meal plus napping and by the exercise are exemples of a phenomenom known as MASKING. The term is intended to illustrate the fact that these changes blurred the value that would occur if no exogenous interference had happened. If you measure your core temperature along several consecutive days these circumstantial variations will end up cancelling each other and you will most probably obtain a curve similar to the "ideal curve".
Whenever you measure a biological variable you will be measuring two components: the rhythmical component (expression of the "biological clocks") and the masking component eventually occurring during your measurement.
Nowadays neuroscience has developed many methods to study biological rhytms and clocks including sophisticated computer programs and statatistical analyses to discover the rhytmical components of organic and behavioral activities. More and more chronobiology is discovering how, when, and why our brain and hormones are modulated by nature's cycles.
Resources on the Internet
Prof. Luiz Menna-Barreto, PhD, Department of Physiology and Biophysics ICB/USP. Grupo Multidisciplinar de Desenvolvimento e Ritmos Biológicos (GMDRB), Institute of Biomedical Sciences - Universidade de São Paulo Email: firstname.lastname@example.org