Sleep and Circadian Rhythms Flashcards
Definition of sleep
A natural, periodic state involving reduced responses to environmental stimuli and decreased mobility
Observed in several species, including unicellular organisms; humans spend 1/3 lives sleeping
What 2 processes is sleep controlled by (Borbely)?
- Homeostatic (accumulation of sleep debt)
- Circadian (typically happens at particular time during 24-hour cycle)
How does sleep differ from a coma?
Different types of brain activity and have more control over sleep.
What is the gold-standard method of studying sleep and what does it involve?
Polysomnography
Involves recordings of electrical activity from multiple sources (obtain an average of brain activity); reveals a ‘sleep architecture’
What 3 main recordings does polysomnography take?
- electroencephalogram (EEG) - activity of neuronal populations beneath the skull
- electrooculogram (EOG) - activity of muscles around eyes to decipher eye movements
- electromyogram (EMG) - activity of muscles in the body
Brain activity during wakefulness
beta waves and alpha waves
- Beta waves consist of irregular activity (13-30Hz); reflects alertness and attention to environmental events or engagement in cognitive processes
- Alpha waves consist of 8-12Hz activity; occurs when quietly resting, no arousal/excitation and not engaged in strenuous mental activity
What activity occurs in stage 1 of sleep?
theta activity
transition between sleep and wakefulness
What activity occurs in stage 2 of sleep?
sleep begins
irregular activity;
* sleep spindles
* K complexes (inhibit activity)
What activity occurs in stage 3 of sleep?
deep sleep
slow-wave sleep (SWS)
high-amplitude and low-frequency delta activity
synchronised, regular activity that is slowing down in conjuction with the slowing down of bodily funcitions (e.g. heart rate)
What activity occurs in REM sleep?
- Rapid Eye Movement (REM)
- theta and beta activity
- increased brain activity and asynchrony in brain waves
- muscle atonia (paralysis)
- ‘paradoxical’ sleep
- dreaming
How long does each cycle last on average?
90 minutes
Typical night of sleep
- spend more time in slow/deep sleep earlier in the night
- spend more time in REM sleep later in the night
Describe the activation-synthesis hypothesis of dreaming by Hobson (2004)
Bottom-up view
* brainstem activated in REM, signals to cortex which produces images with actions + emotions from memory
* frontal cortex less activated, so no logic in timing/sequence of events
* dreams are based on experience but have no meaning
Describe the coping hypothesis (clinico-anatomical hypothesis) by Valli and Revonsuo (2009)
top-down approach
* dreams are biologically adaptive and result in enhanced coping strategies
* dreams are about events people find threatening in their lives
* evidence: problem-solving happens during sleep
What is the role of adenosine in sleep?
Adenosine
* released by astrocytes
* signals depletion of energy
* accummulates during day after prolonged wakefulness and promotes sleep
* antagonised by caffeine
What brain area is associated with sleep induction?
what neurotransmitters does it contain + 2 pieces of evidence
ventro-lateral preoptic area (vlPOA)
* area that induces sleep
* contains inhibitory NTs such as GABA
evidence: damage to area causes insomnia in rats, eventually die
evidence: electirical stimulation causes sleepiness
What brain areas are associated with wakefulness and arousal?
4 areas in total + 1 piece of evidence
reticular formation (reticular activating system; RAS)
* made up of several nuclei in brainstem extending to forebrain, that promote arousal
* locus coeruleus - noradrenaline
* raphe nucleus - serotonin
* tuberomammillary nucleus - histamine
* nucleus basalis of Maynert - acetylcholine
evidence: stimulation of cats’ brainstem caused delta waves to be replaced by beta waves
Describe the flip-flop switch theory (Saper, 2001)
Sleep and wakefulness systems are mutually inhibitory; trying to shut each other down.
* when arousal systems activated, releasing excitatory NTs and inibiting sleep system in vlPOA, so wakefulness
* when sleep system in vlPOA is activated, inhibits arousal systems so slow-wave sleep is possible
What is the role of orexin in sleep?
- released from lateral hypothalamus
- role in maintaining arousal/wakefulness
- implicated in narcolepsy
What are circadian rhythms?
Rhythms or regular patterns of activity associated with a 24-hour cycle
Endogenous cycles (spontaneously generated by brain and body based on the earth’s rotation)
Humans are diurnal (vs nocturnal)
Describe the experiment by Aschoff (1965)
- humans placed in underground bunker with no external cues
- allowed to select day-night cycle (turn lights on and off at will)
- continued showing daily sleep-activity rhythms
- drifted to over 24 hours
= humans have endogenous biological clock governing sleep-wake behaviour
What are exogenous zeitgebers?
external cues that serve to set our biological clock
most important zeitgeber is light
when zeitgeber resets a biorhythm, rhythm has been ‘entrained’
Describe how jet lag is an example of a disrupted circadian rhythm
mismatch between internal circadian clock and external time/cues
results in sleepiness during the day and sleeplessness at night
phase-delays vs phase-advances
What is the brain region associated with circadian rhythms?
suprachiasmatic nucleus (SCN) of the hypothalamus
in experiments, lesions of the SCN disrupted circaidan rhythms of wheel running, drinking and hormonal secretion
What did studies find about the role of the SCN?
- electrode recordings showed greater activity during light periods compared to dark periods
- single cell extracted from SCN and raised in tissue culture continues to function in a rhythmic pattern
- transplantation of SCN into a donor causes recipient to start following donor’s rhythm
How does light reach the SCN?
and how come blind people remain entrained?
- SCN is located above the chiasma of the optic nerves
- receives light information from the retinohypothalamic tract made up of small subgroup of photosensitive retinal ganglion cells
- these PRGCs contain melanopsin (a photopigment) can respond directly to blue light; they don’t rely on rods/cones which explains how blind people remain entrained
