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All living organisms experience rhythmic changes, which tend to coincide with seasonal or daily environmental changes. Most organisms (including humans) have internal biological clocks these are called endogenous pacemakers, which are influenced by external environmental factors called exogenous zeitgebers, to control these periodic changes.
These are rhythms lasting 'about one day'. The best example of a circadian rhythm is the sleep-wake cycle, associated with which are many cyclical changes with active and dormant periods, for example body temperature and urine production. These rhythms allow animals to prepare for predictable daily environmental changes, such as night and day.
Research has involved participants being deprived of possible zeitgebers ('time-givers') like sunrise and sunset, temperature changes during a 24 hour period and wristwatches! Participants tend to maintain a cyclical rhythm but it extends to about 25 hours (Siffre, 1975).
So, endogenous pacemakers can keep a rhythm but exogenous zeitgebers are needed to stick to a 24 hour rhythm.
Where is the brain's internal clock?
The suprachiasmatic nucleus (SCN) in the hypothalamus is a bundle of nerves with an inbuilt circadian rhythm. This is a particularly important endogenous pacemaker. Evidence for this comes from studies in which the SCN has been cut in hamsters to result in disrupted circadian rhythms (Menaker et al., 1978).
What keeps the brain to a 24 hour rhythm?
Without light the brain's day would be 25 hours long (a free-running clock). Light is a very important zeitgeber - flashes of light are enough to 'reset' the internal clocks of animals living in the dark (Aschoff, 1979). One blind man needed to take stimulant and tranquilizing drugs to maintain a 24 hour cycle!
How does light ensure mammals maintain a 24 hour rhythm?
The pineal gland in the brain converts the neurotransmitter serotonin into the hormone melatonin.
Melatonin is released into the blood stream and causes rhythmic changes around the body. Although the need for sleep is not affected by light, melatonin plays a role in the coordination of the sleep-wake cycle.
The animation below shows parts of the brain and neural pathways important in maintaining circadian rhythms.
The absence of light causes melatonin to be released:
These are rhythmic cycles with a period of less than one day. Examples include levels of alertness throughout the day and the cycle of brain activity during sleep.
The use of an electroencephalogram (EEG) can show the electrical activity of the brain. There are different patterns of activity at different times during sleep (Rechtschaffen & Kales, 1968).
The table below gives some information about the different stages of sleep:
|Stage of sleep:||Characteristics:||EEG trace:|
|Non-REM Stage 1.||Theta waves, slow rolling eyes, easily awoken, duration: about 10 minutes.|
|Non-REM Stage 2.||Irregular trace, some theta waves, sleep spindles and K-complexes, duration: about 15 minutes.|
|Non-REM Stage 3.||Deep sleep, high amplitude delta waves (<50% of trace).|
|Non-REM Stage 4.||Deep sleep, high amplitude delta waves (>50% of trace).|
|REM.||Beta waves, paralysis, eye movement, vivid and visual dreaming.|
Here is a graph of a typical night's sleep:
Here are some important points to note:
- Each cycle lasts for about 90 minutes.
- The amount of Stage 3 & 4 sleep decreases each cycle.
- The amount of REM sleep increases each cycle.
These are rhythms with a period of greater than a day. The menstrual cycle is an example of an infradian rhythm. Infradian rhythms that occur as a result of seasonal changes, for example, migration and hibernation are called circannual rhythms.
The menstrual cycle has a period of about 28 days, although the timing can vary according to environmental factors. For example, menstrual cycles may become synchronised if women spend a lot of time together, possibly due to the effect of pheromones passing between them.
Circannual rhythms are under the control of body clocks. But, again, light makes them run on time by influencing melatonin levels.
It is how much light or the daylength, (known as the photoperiod) that provides the useful information about the changes in the seasons; lengthening days mean Spring and Summer are coming and shortening days predict the onset of Autumn and Winter.
In humans, the increased levels of melatonin in autumn appears to lead to a form of depression known as SAD (seasonal affective disorder). This can be treated by exposure to a bright light for several hours per day.
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