Research has involved participants being deprived of possible zeitgebers like sunrise and sunset, temperature changes during a 24 hour period and wristwatches! But again, light makes them run on time by affecting melatonin levels.

Introduction Introduction

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.

All organisms undergo rhythmic changes, which often coincide with seasonal or daily environmental changes. Most organisms (including humans) have internal biological clocks, which are called endogenous pacemakers, and are influenced by external environmental factors, called exogenous pacemakers to control these periodic changes.

Circadian rhythm

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.

These are the rhythms that last "about a day". The best example of a circadian rhythm is the sleep-wake cycle, which is associated with many periodic changes in active and dormant periods, such as body temperature and urine secretion. These rhythms allow animals to prepare for predictable changes in daily environments, such as nights and daytime.

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).

study showed that participants were deprived of possible media ("time provider"), such as sunrise and sunset, temperature changes over 24 hours and watches! Participants tend to maintain a periodic rhythm. Participants tend to maintain a periodic 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.

Therefore, endogenous pacemakers can maintain a rhythm, but exogenous zeitgebers are needed to stick to a 24 hour rhythm.

Where is the brain's internal clock? Where is the internal clock of the brain?

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).

The upper nucleus (SCN) in the hypothalamus is a bunch of nerves with an intrinsic circadian rhythm. This is a particularly important endogenous pacemaker. The evidence for this comes from some studies that cut off SCN in hamsters, resulting in circadian rhythm disorders (Menaker et al., 1978).

What keeps the brain to a 24 hour rhythm? 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!

If there is no light, the brain's day will be 25 hours (a free-running clock). Light is a very important medium - flash is enough to "reset" the internal clock of animals living in the dark (Aschoff, 1979).A blind person needs to take stimulants and sedatives to maintain a 24-hour cycle! This is light.

How does light ensure mammals maintain a 24 hour rhythm?

The pinealgland in the brain converts the neurotransmitter serotonin into the hormone melatonin.

pineal gland converts the neurotransmitter 5-hydroxytryptamine to 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.

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.

Ultradian rhythms

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.

These are rhythmic cycles with a period of less than one day. Examples include alertness levels throughout the day and brain activity cycles 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).

Use Electroencephalogram (EEG) can show the electrical activity of the brain. During sleep, different activity patterns are present at different times (Rechtschaffen & Kales, 1968).

The table below gives some information about the different stages of sleep The following table gives some information about the different stages of sleep:

Here is a graph of a typical night's sleep Below is a typical night's sleep :

Here are some important points to note Here are some important points to note:

1. Each cycle lasts for about 90 minutes.
2. The amount of Stage 3 & 4 sleep decreases each cycle. The amount of sleep in the third and fourth stages decreases in each cycle.
3. The amount of REM sleep increases each cycle. Rapid eye movement sleep increases in each cycle.

Infradian rhythms

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.

These are rhythms whose cycles are greater than one day. The menstrual cycle is an example of the next day rhythm. Noncircadian rhythms that occur due to seasonal changes, such as migration and hibernation, are called circular annual 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.

The menstrual cycle has a period of about 28 days, although the timing can vary according to environmental factors. For example, if women are often together, the menstrual cycle may become synchronized, which may be due to the influence of the transmission of pheromones between them.

Circannual rhythms are under the control of body clocks. But, again, light makes them run on time by influencing melatonin levels.

Periodic rhythms are under the control of body clocks. But again, light makes them run on time by affecting 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 means Spring and Summer are coming and shortening days predict the onset of Autumn and Winter.

is how much light or the daylengthing time (called light cycle) provides useful information about the changes in the seasons; extended sunshine time means spring and summer, and shortening sunshine time indicates the arrival of autumn and winter.

In humans, the increased levels of melatonin in autumn appear 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.

In humans, the increased levels of melatonin in autumn appear 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.

In humans, the increased levels of melatonin in autumn appear 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.

In humans, the increased levels of melatonin in autumn appear 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.

In humans, the increased levels of melatonin in autumn appear 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.

In humans, the increased levels of melatonin in autumn appear 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.

In humans, the increased levels of melatonin in autumn appear 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.

In humans, the increased levels of melatonin in autumn appear to lead to a form of depression known as SAD (seasonal affective This can be treated by exposure to bright light for several hours a day.