Biological Clocks Linked to Health

Cape Times (South Africa), April 20, 2007 | Go to article overview

Biological Clocks Linked to Health


BYLINE: Laura Roden

Many people find it more difficult to get up for work on a Monday than the other days of the week. Others struggle to adjust to night shift after working days or vice versa. Jet lag can leave travellers with indigestion or feeling really lousy after having just one alcoholic drink on that first "night" in a new time zone.

The reason for these experiences is the disruption and desynchronisation of biological rhythms with environmental rhythms.

Biological rhythms are not to be confused with biorhythms: biorhythms are to biological rhythms as astrology is to astronomy.

In humans, metabolism, hormone secretions, the expression of some genes, mental alertness, manual dexterity, athletic performance and social interactions all show daily rhythms.

These rhythms have periods of about 24 hours and are called circadian from the Latin circa meaning "about" and dies meaning "day".

A defining feature of all living organisms is that they display biological rhythms, and lots of them, with durations of milliseconds to hours, days, months and some of several years.

The study of biological rhythms and time keeping is called chronobiology. Scientists have discovered the molecular and genetic bases for some of these rhythms in many organisms and are now realising their importance in human health.

There is a rapidly emerging field of chronomedicine in which the differing effects and efficacy of drugs or treatments depending on the time of administration is studied.

Time-specified treatment can vastly improve the effectiveness of the drug or treatment and reduce the potential side effects because the way it is metabolised and what is being exposed to it will differ at different times of the day.

Our solar day, the time it takes for our planet to rotate on its axis, is 23 hours and 56 minutes, or about 24 hours. The most obvious effect of the Earth's rotation is a rhythmic change in light and dark, and, because of the tilt of the Earth on its axis, a rhythmic change in seasons as it orbits the sun.

All life on Earth has developed in the presence of these rhythms, and many organisms, from single-cell bacteria to plants and humans, have acquired the ability to anticipate these changes in the environment and accordingly alter their behaviour, metabolism and physiology in a rhythmic fashion.

We are so good at anticipating the changes that if we are put into an environment devoid of external cues of light and temperature, we continue to behave as if the changes were still occurring. For instance, if we visit Antarctica in the middle of summer when the sun doesn't set, or if we spend several days stuck in a deep, dark cave, our sleep-wake cycle, the most obvious of our daily rhythms, would persist and with a roughly 24-hour period.

We have an internal clock housed at the base of our brains in a structure called the suprachiasmatic nuclei or SCN, which drives rhythms in our bodies and synchronises our internal time with the external environment.

This synchronisation uses mainly light as a cue and can be thought about as similar to adjusting a slow or fast wrist watch each day. Instead of cogs and winders or electronics, gene expression and hormone secretions are stimulated or inhibited to reset our internal clocks.

We notice the effects of our internal body clock governing our physiology and how we feel when we cross several time zones or engage in shift work. …

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