Biological rhythm cycles vary in length:

Biological rhythms can be classified according to how long their cycle lasts.

1. Circadian rhythms- have cycles that generally occur once every 24 hours. For example, we usually go through the sleep-wake cycle once every day

2. Infradian rhythms- have cycles that occur less than once every day. For example, the menstrual cycle. Sabbagh and Barnard (1984) found that when women live together their menstrual cycles may synchronise. It isn’t clear why, but it may be linked to pheromones (chemicals that can affect the behaviour or physiology of others)

3. Ultraradian- have cycles that occur more than once every 24 hours. For example, the sleep cycle has repeating stages of light and deep sleep. Research using EEGs to monitor brain activity during sleep has shown that a regular sleep pattern is really important. Disrupting these cycles can have serious consequences

Biological rhythms are regulated by Internal and External influences:

The timing of biological rhythms is determined by factors both inside and outside our bodies.

Endogenous pacemakers:

1. Some aspects of our biological rhythms are set by genetically determined biological structures and mechinism within the body

2. The suprachiasmatic nuclues (SCN), part of the hypothalamus, seems to act as an internal clock to keep the body on an aproximate 24-hour sleep-waking cycle

3. It is sensistive to light and regulates the pineal gland, which secreates melatonin- a hormone which seems to sleep. When there is less light, more melatonin- a hormone which seems to induce sleep. When there is less light, more melatonin is produced. When there is more light, secretion is reduced and waking occurs

4. Menaker et al (1978) lesioned this structure in hamsters- their sleep-waking cycle was disrupted

Exogenus zeitgebers:

1. These are influences outside of the body that act like a prompt, which may trigger a biological rhythm

2. Light is the most important zeitgeber. Siffre (1975) spent six months in a cave. He had no clocks and no natural light as zeitgebers. His sleep-walking cycle extended from 24-hour to a 25-30 hour cycle. It therefore seems that natural light is needed to fine-tune our normal 24-hours cycle

Edogenous and Exogenous factors interact:

Endogenous and exogenous factors interact to regulate the timing of our biological rhythms

1. In some cases, endogenous factors may completely determine a cycle. Pengelly and Fisher (1957) found that squirrels will hibernate even when kept in laboratory conditions very different from their natural environment

2. However, many animals can react more flexibly, especially humans who are able to adapt to their surroundings. We can make ourselves stay and change the environment to suit our needs, e.g. by using artificial light

3. Cultural factors are also important. For example, Eskimos often live in permanent daylight or permanent night-time but can maintain regular daily sleep cycles- so the cycle can’t just be determined by levels of light acting on the pineal gland

4. Individual differences can also affect the rhythms. Aschoff and Weaver (1976) found that in a group of people isolated from daylight, some maintenaed their regular sleep-wake cycles. Other members of the group displayed their own very extreme idiosyncrasies, e.g 29 hours awake followed by 21 hours asleep. This also shows that factors must interact to control or influence biological rhythms

Disrupting biological rhythms can have negative consequences:

When endogenous pacemakers become out of line with exogenous zeitgebers, it can disrupt the sleep-wake cycle.

1. In the natural environment, zeitgebers normally change slowly, e.g light levels during the year change gradually

2. However, in modern society, zeitgebers can change quickly.This can have negative effects on our ability to function- slowing reaction times, imparing problem-solving skills, and limiting our ability to concentrate

Jet lag:

1. Jet planes allow fast travel to different time zones. Leaving the UK at 9am means that you’d get to New York at about 4pm UK time. New York is 5 hours behind the UK, so the local time would be 11am

2. Consequently you’ll feel sleepy at an earlier (local) time. If you then went to sleep you would wake up earlier and be out of sync with local timing. It appears easiest to adapt by forcing yourself to stay awake

3. It can take about a week to fully syncronise to a new time zone. Wegman et al (1986) found that travelling east to west (phase advance)

4. Schwartz et al (1995) found that baseball teams from the east coast of the USA got better results traveling to play in the west did when travelling to play in the east

Research on biological rhythms has limitations:

1. Findings from animal studies can’t acuratly be generalised to humans- humans have greater adaptability

2. Studies that have deprived humans of natural light have still allowed artificial light, which may give many of the benefits of natural light- this reduces the validity of these studies

3. Things like individual differences need further study. Some people are more alert early in the day, and others later on, and the speed with which we adapt to disruption can vary. It’s difficult to determine whether person’s lifestyle is cause or effect of their biological rhythms

4. If we fully understand what causes the problems linked to jet lag and shift work, we can minimise or avoid them, reducing accidents in work environments. There are different ways to deal with these problems, e.g. taking time to naturally adjust, or using drugs to reduce the effects of sleep deprivation