Circadian Rhythm
The sleep to waking cycle which takes place over the course of a day (around 24 hours)
Zeitgebers
- Environmental cues which control the timing of circadian rhythms
- The daily light-dark cycle
- Not necessarily 24 hours - we can engineer our circadian rhythms by changing light-dark exposure
- The daily light-dark cycle
Free-running rhythms
Circadian rhythms that continue to exist in environments with no environmental cues
Free-running period
The duration of free-running rhythms
Generally a little longer than 24 hours (our biological clocks run a little slow without environmental cues)
Internal desynchronization
- Individuals in the same, constant environments do not always share sleep-wake and body temperature cycles
- Free-running period can change seemingly randomly
Suprachiasmatic Nuclei
Contains a circadian timing mechanism
an area in the medial hypothalamus supposedly controlling our circadian cycles
- Timing mechanisms are controlled by the firing patterns of SCN neurons
- Inactivity at night
- Fire at dawn
- Fire slowly & steadily all day
Circadian Clock
Circadian cycles are controlled by internal timing mechanisms, free-running rhythms provide evidence for this
Retinohypothalamic tracts
Visual axons crucial in entraining light-dark driven circadian rhythms
Convey information about how much light there is in the environment
Retinol ganglion cells
- Key to entraining circadian rhythms
- Evolved to be sensitive to slow changes in background illumination
Melatonin
- Produced in the pineal gland
- Releases a melatonin to help influence circadian rhythms
- Synthesized from serotonin
- Adjusts the timing of internal biological clock
- Chronobiotic
Recuperation Theories
We sleep to restore homeostasis, which was disturbed while awake
Most common/Main theories
1. We sleep to restore energy depleted while awake
2. We sleep to clear toxins from the brain built up while awake
- We wake when homeostasis has been achieved
Adaptation Theories
We sleep as a result of our internal, 24-hour body clocks, regardless of events during wakefulness
Evolutionary
- We slept to conserve energy and because we functioned less well in the dark
Sleep deprivation
Being unable to return to homeostasis affects stress, concentration, memory, mood & ability to complete complex tasks
Stage 1 Sleep (NREM 1)
Theta waves - irregular, jagged, low-voltage brain waves
4-7 Hz
Brain activity - less relaxed than wakefulness but more so than other sleep stages
Stage 2 Sleep (NREM 2)
K-complex
Sleep spindle
Sleep spindle
a burst of 12- to 14-Hz brain waves associated with consolidation of memory
1. Result from interactions between cells in the thalamus and cortex
2. More spindles = improvements to memory
K-complex
sharp wave associated with temporarily inhibiting neuronal firing
Slow Wave Sleep (NREM 3)
- Delta waves
- Decrease in heart rate, breathing rate and brain activity
- Increase in slow, large-amplitude waves
- Indicates high synchronization in neuronal activity
- Compared to stage 1 & being awake where substantial activity in the cortex is continued
REM Sleep
Rapid eye movement sleep
- Emerging stage 1
- REM sleep occurs when we’ve cycled back to stage 1
Elements of light and deep sleep
Increase in cerebral activity
Increase in variability of autonomic nervous system
- Strengthening of emotional memories
- Where most dreaming occurs
Default Theory of REM Sleep
Difficult to stay in REM sleep so the brain switches to other stages throughout the night)
Activation Synthesis Hypothesis (Dreams)
Random information & neural signals supplied to the cortex during REM so it forms dreams to make sense of them
Neurocognitive Hypothesis
- Dreams are thoughts taking place under unusual conditions
- Parts of the parietal, occipital and temporal cortex experience stimulation which develops into a hallucination
- No input from regular sensory sources means the brain generates images without context
- Forgetting dreams & forgetting within dreams - suppression in the prefrontal cortex (working memory)
Reticular Formation
A structure extending from the medulla into the forebrain which regulate arousal
- Pontomesencephalon - part of reticular formation contributing to cortical arousal
- Neurons receive sensory input & generate activity
Locus Coeruleus
A small structure in the pons which emits bursts of impulses at emotionally arousing/meaningful events
- Releases norepinephrine throughout the cortex
- Increases activity in more active neurons, decreases activity in less active neurons
- This results in enhanced memory & attention to important information
