Rhythms Flashcards
Biological rhythms
All living organisms - plants, animals and people- are subject to
biological rhythms. Biological rhythms are cyclical changes in the way
that biological systems behave. These rhythms have evolved because the
environment in which organisms live has cyclic changes.
Endogenous pacemakers
The body’s internal biological ‘clock’ – The
Superchiasmatic Nucleus (SCN)
Exogenous zeitgebers
External changes in the environment. Eg light or
social events like meal times.
Ultradian Rhythms
Take less than 24 hours
E.g. the Sleep Stages cycle
Infradian Rhythms
Take longer than 24 hours
E.g. the menstrual cycle
Circadian Rhythms 1
Take just 24 hours
E.g. the Sleep –wake cycle
Circadian Rhythms 2
‘Circa’ meaning about and ‘diem’ meaning day
Most important type of circadian rhythm is the sleep/ wake cycle.
Sleep/wake cycle is governed by both internal and external mechanisms .
External – Exogenous zeitgebers (e.g. daylight)
Internal – Endogenous pacemakers – internal bio clock ‘left to its own devices’ without the influence of external stimuli (e.g. light) is called ‘FREE-RUNNING’
The sleep/wake cycle is controlled by the
SUPRACHIASMATIC NUCLEUS (SCN) – nerve fibres connected to the eye cross in an area called the optic chaism (OC) on their way to the visual area of the cerebral cortex.
The SCN lies just above the OC (thus ‘Supra’ means above).
Circadian Rhythms 3
This ‘free-running’ internal
circadian clock maintains a cycle of
24-25 hours even in the absence of
external cues. It is disrupted by
major changes in sleep schedules
such as jet travel and shift work.
The SCN receives info about light
directly from this structure
The EZ (light) can reset the SCN
Serotonin vs Melatonin
Serotonin and melatonin perform opposite jobs and yet they must work in harmony to keep the body balanced.
Serotonin is a feel-good hormone that increases positivity and relaxation, as well as helping providing energy. This
energy is particularly important because it is needed to get going in the morning .
Melatonin is the hormone of darkness. The nickname refers to the fact that melatonin is produced in the pineal gland of
the brain when in a dark environment. The change in light causes messages to be sent from the ganglion cells in the eye
the optic chiasm and activate the SCN within the hypothalamus, which instructs the pineal gland to produce more
melatonin. The melatonin winds the body down to a more lethargic and sleep-ready state.
Without melatonin, it would be impossible to achieve relaxed, restful sleep and so the body would not be able to go
through the restorative processes that typically take place in bed.
So what does this mean in terms of sleep?
Levels of melatonin are boosted when it’s dark, whereas serotonin levels increase in sunshine and light environments.
In short, melatonin helps you get to sleep and serotonin helps you feel awake when you get up the next day.
A lack of melatonin can cause sleeplessness and even insomnia, whereas a deficiency in serotonin can result in feelings
of depression and lethargy.
Melatonin
A hormone that is produced by the pineal gland in animals and regulates sleep and
wakefulness. It does this by inhibiting the brain mechanisms that promote wakefulness.
Body temperature
Body temperature fluctuate
from 36 – 38 degrees.
The lowest temperature is a
4:30 AM and highest at round 6
PM.
There is a slight trough at
around 3pm in the afternoon
(may link to feeling sleepy)
CASE STUDY: Michel Siffre
Sifre (1975) was a speleologist (cave expert) who investigated the natural
duration of his own sleep-wake cycle. He isolated himself from all daylight
by hiding in a dark cave with only weak artificial light for 6 months. Siffre
had no clock or any reference to time, however he did have verbal contact
with the outside world. The findings were that his sleep-wake cycle settled
naturally at around 25 hours, although sometimes it would range up to 48
hours. After 179 days had passed he thought 151 had passed.
Siffre concluded that the absence of natural daylight allowed his biological
clock to run at its natural rate, and that normally daylight acted as an
exogenous zeitgeber to resynchronise the cycle.
The study was, of course, a case study and cannot therefore be readily
generalized to other people, however it has been repeated several times by
other researchers.
Aschoff and Wever (1976) - WW2 Bunker
Placed ppts in an underground WW2 bunker in the absence of environmental and social times cues. They found that most people displayed circadian rhythms between 24-25 hours, though some rhythms were as long as 29 hours.
Conclusion Case Studies:
These studies seem to suggest that circadian rhythms persist despite isolation from natural light, which demonstrates the existence of an endogenous ‘clock’. However, this research also shows that external cues are important because the clock was not perfectly accurate: it varied from day to day.
Limitations with such research – Czeisler
Studies such as these may have a large methodological flaw which may mean that the results may not be useful in explaining the role of circadian rhythms in humans and what factors play a role in their regulation.
In all studies, participants were isolated from variables that might affect their circadian rhythms such as clocks, radios and daylight, in.
However, they were not isolated from artificial light because it was thought that dim light, in contrast to daylight, would not affect the circadian rhythm.
Recent research by Czeisler et al (1999) suggests that this may not be true as they altered participants’ circadian rhythms down to 22 hours and up to 28 hours just using dim lighting.
Suggesting that the results obtained from this previous research may lack validity and therefore only give a limited amount of knowledge about circadian rhythms.
