PSYA01H3 Chapter Notes - Chapter 5.2: Ultradian Rhythm, Circadian Rhythm, Suprachiasmatic Nucleus
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5.2 Biological Rhythms of Consciousness: Wakefulness and Sleep
o a person’s awareness, including thoughts, perceptions, experiences of the world
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.
▪ 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
- 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
o from research, most people fell into a 24.5-hour circadian rhythm
- The Stages of Sleep
▪ 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-
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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