This is the first instalment of a three-part series about the circadian clock that controls your sleep cycle, hormone levels and body temperature.
Biological clocks
Planet earth has its rhythms. It orbits the sun once every 365 days giving rise to the changing seasons, and spins on its axis every 24 hours giving rise to day and night.
Almost all living things have internal clocks that enable them to synchronise their behaviour and physiological processes to nature's rhythms.
Evidence for the existence of biological clocks goes back a long way. In the 1700s, French scientist Jean Jacques Ortous De Mairan was curious to find out what made the leaves of plants rigid during the day but limp at night.
He placed a mimosa plant in a dark cupboard and was surprised to observe that its leaves continued to droop and stiffen with a daily rhythm despite the absence of daylight.
The plant's leaves were not merely reacting to changing light levels, but appeared to be controlled by some kind of inbuilt timer.
The common cockroach is most active during the first few hours of darkness. But studies have shown that cockroaches kept in constant darkness and at a controlled temperature continue to concentrate their activity into two- to three-hour bursts, which recur approximately every 24 hours.
Their behaviour is driven by an internal clock that mimics the day-to-night rhythm of the outside world.
Not only the mimosa and the cockroach, but practically all living creatures show daily cycles in their physiological processes. These are known as circadian rhythms (from the Latin: circa, about; diem, a day).
In mammals, the location of the body's master clock has been narrowed down to a clump of about 20,000 cells in the suprachiasmatic nuclei (SCN), a part of the hypothalamus at the base of the brain.
The cells in the SCN produce a rhythmic output of electrical and hormonal signals that drive the animal's circadian rhythms.
The circadian clock in humans
In humans, as in other mammals, the circadian clock ensures that the body's organs and systems work in harmony with one another and in sync with the external world.
This includes telling each organ when it must work hardest and when it can take it easy. For example, kidney function is reduced at night so that we do not need to urinate while we sleep.
Most of our physiological functions vary to some degree depending upon the time of day or night.
Body temperature, blood pressure, heartbeat and hormone levels all show daily changes. Our moods, levels of alertness, physical performance and mental abilities also dip and peak at predictable times.
We humans are diurnal creatures: we sleep at night and are active during the day. The daily switch from waking to sleeping is tightly choreographed by our circadian clocks so that we wake and sleep at the appropriate times.
For most of us, sleepiness reaches a maximum somewhere between 9pm and 10pm. At this time, our body temperature begins to drop, and is about one degree lower when we are asleep than it is when we are awake.
At about 4am our body temperature starts to rise again in preparation for the start of a new day.
In one study, volunteers were sent deep underground for a prolonged period of time. In the absence of daylight their bodily rhythms settled into an approximately 25-hour cycle.
Two weeks into the study they were going to bed around midday and getting up in the early evening. After two more weeks, their sleep patterns came back in sync with the outside world, before drifting away again.
In another study, volunteers were deprived not only of daylight, but also electric lighting and all other external time cues.
Their bodies started to run on a 24-hour 11-minute cycle - slightly longer than the 24-hour period of the earth's rotation. Our circadian clocks run a little slow.
External cues, principally the coming of daylight and the onset of dusk, are used to reset our circadian clocks and keep us in sync with day and night.
The process is analogous to using a wristwatch that runs a bit slowly. But by adjusting it each morning, it matches an accurate time-signal.
The modern world
Our biological clocks are the result of three billion years of evolution.
They help us anticipate and respond to nature's rhythms. But in the modern world these natural rhythms are disrupted by electric lights, alarm clocks and the demands of a 24-hour society.
Next week's article will look at how our circadian clocks cope - and sometimes fail to cope - with these new demands.
