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Chapter 5.2

PSYA01H3 Chapter Notes - Chapter 5.2: Ultradian Rhythm, Circadian Rhythm, Suprachiasmatic Nucleus

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Steve Joordens

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5.2 Biological Rhythms of Consciousness: Wakefulness and Sleep
- Consciousness
o a person’s awareness, including thoughts, perceptions, experiences of the world
and self-awareness
o we go through many changes in consciousness every day as our thoughts and
perceptions constantly adapt to new situations
- organisms have evolved biological rhythms are neatly adapted to the cycles in their
- Circannual Rhythm (yearly cycle)
o ex. hibernation
- Infradian Rhythm
o occurs over period of time longer than a day
o ex. menstrual cycle
- Ultradian Rhythm
o more frequent biological rhythms
o ex. hormonal activity occurs in 90-120 minutes cycles
- Circadian Rhythm
o internally driven daily cycles of approximately 24 hours affecting physiological
and behavioural processes
o involve the tendency to be asleep or awake at specific times, feel hungrier at
parts of the day and the ability to concentrate better at certain times than others
o Suprachiasmatic Nucleus (SCN) of the hypothalamus
a key brain structure in this process
Light is the primary stimulus regulating the human circadian rhythm.
Cells in the retina of the eye relays messages about environmental light
levels to the suprachiasmatic nucleus (SCN) of the hypothalamus.
The SCN communicates signals to the pineal gland, causing it to release
the hormone melatonin, which peaks in concentration at nighttime and is
reduced during wakefulness
o Circadian rhythms change with age. We need much less sleep especially REM
sleep as we get older.
o People also tend to experience a change in when they prefer to sleep.
o Entrainment
when biological rhythms become synchronized to external cues such as
light, temperature, or even a clock
light is a primary entrainment mechanism because of its effects on the
SCN-melatonin system
- Endogenous Rhythms
o biological rhythms that are generated by our body independent of external cues
such as light

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o it is difficult to study because it is difficult to remove external cues from a
person’s world
o from research, most people fell into a 24.5-hour circadian rhythm
- The Stages of Sleep
o Polysomnography
refers to a set of objective measurements used to examine physiological
variables during sleep
o Sleep cycles are most often defined by the electroencephalogram (EEG)
These waves can be described by their frequency the number of up-
down cycles every second
and their amplitude the height and depth of the up-down cycle.
Beta waves
high-frequency, low-amplitude waves are characteristics of
often interpreted as a sign that a person is alert.
Alpha waves
signal that a person may be daydreaming, meditating, or starting
to fall asleep
as the individual begins to shift into sleep, the waves start to
become slower, larger, and more predictable
o The EEG signals during sleep move through four different stages
Stage 1
brain waves slow down and become higher in amplitude these
are known as theta waves
Breathing, blood pressure, and heart rate all decrease slightly as
an individual begins to sleep.
After approximately 10 to 15 minutes, the sleeper enters stage 2,
during which brain waves continue to slow.
Stage 2
includes sleep spindles and K complexes, which are detected as
periodic bursts of EEG activity
o sleep spindles: clusters of high-frequency but low-
amplitude waves)
o k complexes: small groups of larger amplitude waves
the meaning of the bursts are not completely understood but
evidence suggests they play a role in helping maintain a state of
sleep and in the process of memory storage
as stage 2 progresses, we respond to fewer and fewer external
stimuli, such as lights and sounds
Approximately 20 minutes later, we enter stage 3 sleep
Stage 3
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