Biological rhythms
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Biological rhythms
Regular variations in the biological activity
of living organisms, such as sleep, body
temperature, alertness, neurotransmitter
levels, etc.
 Ultradian rhythms: less than 24 hrs
periodicity
 Circadian rhythms: 24 hrs periodicity
 Infradian rhythms: more than 24hrs
periodicity
 See page 2 – identify which rhythms
 Try experiment on page 3

Factors which affect biological
rhythms
Endogenous pacemakers: Biological
‘clocks’ in the brain controlling biological
rhythms. Most likely the suprachiasmatic
nucleus in the hypothalamus
 Exogenous Zeitgebers (Timegivers):
External stimuli that help towards
regulating biological rhythms to the
outside world
 Think of some examples.

Circadian rhythms
Examples:
sleep/waking, body
temperature
 Circadian rhythms
are needed to
balancing behaviour
and body states to
environmental
changes

The
Biological
Clock – how
does it work?
The Biological Clock - How does it work?


Thought mainly to be an endogenous
(internal) mechanism
Our internal rhythms are thought to be
generated by protein synthesis within
the SCN. Protein is produced for a
period of hours until it reaches a level that
inhibits further production. Over the next
few hours the protein level gradually falls,
when it drops to a certain ‘threshold’ level
then production of the protein re-starts.
This generates an internal (endogenous)
biological rhythm – in humans of between
24 ½ and 25 hours. See textbook
This endogenous rhythm is regulated to keep it
in line with our environmental rhythms by an
exogenous mechanism below:
SCN have a nerve input directly from the retina of
the eye, so they are kept informed about the
zeitgebers of light and darkness.
I
The Pineal Gland
Important pacemaker in birds and reptiles.
Lies just beneath the bone of the skull &
receives info about light.
Many lizards have a ‘third eye’ near the
pineal gland, which
protrudes through a
small opening in the
skull.
In darkness this
information is
transmitted to the
pineal gland which
manufactures
melatonin a hormone
which stimulates the
production of
serotonin in the raphe
nucleus. This hormone
causes reduced levels
of arousal and so
induces sleep.
1) Darkness Falls
2)Optical Chiasm
3) Stimulates SCN
4)Stimulates Pineal Gland
5) Produces Melatonin
6) Produces Seratonin
7) Brain Activity Falls
8) Sleep
When Light enters
through the eyes it
stimulates the SCN
to produce an
inhibiting factor
(probably a protein)
This inhibits the
production of
melatonin by the
pineal gland and so
maintains arousal
levels – keeping you
alert and awake!
Answer questions on page 8-9
Test
Explain how one exogenous zeitgeber
affects the sleep/wake cycle
What type of rhythm is the sleep/waking
cycle?
Explain why the pineal gland is an
important part of the biological clock?
Explain why the mechanism for
birds/reptiles and humans/mammals is
different.
Give an example of an ultradian rhythm
Siffre study

Michel Siffre in 1972 went to live in a cave
for 6 months. When he was awake the
lights in the cave were on, when he went
to bed the lights were off. His sleep/wake
cycle regulated between 25 and 30 hrs
(more than 24 hrs). When he reached the
179th day, his days were only 151 since he
started living underground
Morgan (1995)
Bred mutant hamsters so they had
circadian rhythms of 20 instead of 24
hours. He transplanted their SCN into
normal hamsters.
 The normal hamsters displayed the
‘mutant’ rhythm.
 What does this provide evidence of?

Miles et al (1977)
Miles et al’s (1977) study of
a blind man. The man blind
from birth had a circadian
rhythm of 24.9 hours. He
had to use stimulants and
sedatives to adjust his sleepwaking cycle to the standard
24 hours. This shows that
light is the main exogenous
factor, as it reduces the
natural 25 hour rhythm to 24
hours.
Evidence - The Role of Melatonin
Schochat et al’s “Sleep gate” study (1997) found
a close relationship between sleep propensity
and melatonin. Six participants spent 29 hours in
a sleep laboratory where for 7 minutes in every
20 they had to try to sleep. The highest sleep
propensity, the “sleep gate” was found to occur
in late evening and the highest levels of
melatonin preceded this by 100-120 minutes.
But this study only shows a correlation!
Infradian rhythms


Occurring to a period
more than 24 hrs, such
as the human
menstrual cycle
Some of them may
take place on a yearly
basis and they are
known as circannual
rhythms, such as the
migration of birds, and
hibernation in squirrels
bears, and hedgehogs
The
Menstrual
Cycle -an
example of an
Infradian
Rhythm, is
controlled by
hormones
oestrogen
and
progesterone
Menstruation is an endogenous
rhythm but similar to the sleep
wake cycle research has shown
that it can be affected by many
exogenous cues. – RESEARCH :-
A woman who spent 3 months in
a cave developed a shortening
of her menstrual cycle which
took some time to revert back to
normal once she emerged
(Reinberg, 1967)
Infradian rythms

Menstruation in the
absence of Zeitgebers: It
has been observed that
menstruation starts
mainly in winter (for the
first time), and that
because the pineal gland
is affected by the
melatonin’s secretion
and the reproduction
system in general
(conceptions in lighter
months) (Reinberg,
1967)
Russell et al 1980 found that pheromones –
biochemical substances that act like
hormones but are released into the air can
have the effect of synchronising a group of
women’s cycles upon repeated exposure to
one another.
McClintock (1971) synchronised cycles
through pheromones of other women &
synchronised cycles through pheromones of
men
Infradian rhythms

Seasonal affective disorder (SAD):
Seasonal changes in behaviour can also be
found in human beings. Individuals feel
depressed during winter, and elated
during summer. One suggestion is that
absence of light increases melatonin, and
what is suggested is light therapy
(Rosenzweig et al., 1999). As an example,
depression is mostly seen in northern
than southern countries instead
Ultradian rhythms

Shorter than 24 hrs,
such as smoking, eating
and drinking, renal
excretion (discharging
waste through
kidneys), sleep
(different stages
studied through EEGElectroEncephalography), etc.
(Loomis et al., 1937)
Ultradian rhythms (Recording Sleep)
EEG (Electro-EncephaloGraphy): It
measures and records the electrical
activity in a person’s brain
 EOG (ElectrOculoGram) it records eye
movements during sleep
 EMG (ElectroMyoGram) it records
muscle activity during sleep

Test





Outline what is meant by an endogenous
pacemaker
Why is the hormone melatonin important?
Briefly describe a study about a ‘free-running’
biological rhythm and what was found.
Describe the findings of Russell et al’s study into
mentrual cycles.
Explain how an exogenous zeitgeber influences
the mentrual cycle.
Luce and Segal Artic Circle study (1966)
People who live within the Artic circle sleep for
7 hours per night despite the fact that during
the summer the sun never sets. This shows
that light is not the only zeitgeber, nor is the
biological clock only influenced by light. Other
exogenous factors such as social customs and
psychological factors

Kleitman (1963): Student partcipants went
to live to an underground bunker, with no
cues of light or dark. They had to choose
their own sleep/wake times. Their
circadian rhythms were extended
between 25 to 27 hrs
Evaluations
Individual differences: Biological rhythms
do not operate the same in all people, or
in subjects of different parts of the world
 Number of participants: Because most of
the research was carried out with single
participants, the findings are generalised
with great care

Description





Term
Rhythm over course of
24 hrs
Rhythm more than a
day
Rhythm less than a day
Internal factors
affecting rhythms
External factors
affecting rhythms





Exogenous Zeitgebers
Endogenous
pacemakers
Ultradian
Infradian
Circadian
Check Key terms
Test
What three things are measured in
studies of sleep cycles
 What did Luce and Segal’s research
suggest about Zeitgebers?
 Suggest some possible methodological
criticisms of research into biological
rhythms
 How do we know that free-running sleep
cycles are different the normal 24 hrs?

Disrupting biological rhythms
Jet lag
Travelling to different parts of the world for some
days after the journey the sleep/wake cycle is
disrupted (body temperature fails/body arousal
may increase or decrease)
 Shift work
Employees required to a shift work, normally go
to sleep when others are awake. When
biological rhythm is disrupted (endogenous
pacemakers/exogenous zeitgebers), sleep/wake
cycle is disrupted too

Consequences
Stevens (2006) Artificial lighting may cause
disruption to biological rhythms which was not
accounted for in some studies e.g Siffre.
 Decreased Melatonin levels is associated with
cancer. This may explain why breast cancer is
more common in western countries where
exposure to artifical lighting is greater. Coren
(1996) argues that because of all those changes
in our lives we end up sleeping 11/2 less than in
a century ago

Consequences
There are a range of consequences
associated with a lack of sleep including
 Poor decision making (see incidents on
page 18)
 Same affect on driving as alcohol
(DeWaard and Brookhuis 1991)
 Female flight attendants performed poorly
on attention tasks compared to grounded
colleagues (Cho et al, 2000)

Evaluation of research on disrupting
biological rhythms
Research findings
Disrupting our biological rhythms has cognitive and
emotional effects, as well as drastic consequences
 Methodological issues
Studies have great ecological validity, however
personality and individual differences are not
controlled in those studies. Most are correlations
e.g between shift work and road accidents ??

Write a shocking news article !
Write the article to inform the public
about the dangers of shift work and jet
lag to our health.
 You may wish to go to the library and
investigate accidents which have occurred
during the night when workers are on
night shift!

Recap
Complete gap fill page 19
 Test

Explain the affect of artificial light on biological
rhythms.
 Explain the affect of lack of sleep on driving
ability.
 Suggest a study which supports the idea that
shift work results in poor performance.
 What affect does SAD have on melatonin
levels?
