Sleep and Circadian Rhythms

Question 1

Definition of sleep

Answer 1

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

Question 2

What 2 processes is sleep controlled by (Borbely)?

Answer 2

• Homeostatic (accumulation of sleep debt)
• Circadian (typically happens at particular time during 24-hour cycle)

Question 3

How does sleep differ from a coma?

Answer 3

Different types of brain activity and have more control over sleep.

Question 4

What is the gold-standard method of studying sleep and what does it involve?

Answer 4

Polysomnography

Involves recordings of electrical activity from multiple sources (obtain an average of brain activity); reveals a ‘sleep architecture’

Question 5

What 3 main recordings does polysomnography take?

Answer 5

• 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

Question 6

Brain activity during wakefulness

beta waves and alpha waves

Answer 6

• 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

Question 7

What activity occurs in stage 1 of sleep?

Answer 7

theta activity

transition between sleep and wakefulness

Question 8

What activity occurs in stage 2 of sleep?

sleep begins

Answer 8

irregular activity;
* sleep spindles
* K complexes (inhibit activity)

Question 9

What activity occurs in stage 3 of sleep?

deep sleep

Answer 9

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)

Question 10

What activity occurs in REM sleep?

Answer 10

• Rapid Eye Movement (REM)
• theta and beta activity
• increased brain activity and asynchrony in brain waves
• muscle atonia (paralysis)
• ‘paradoxical’ sleep
• dreaming

Question 11

How long does each cycle last on average?

Answer 11

90 minutes

Question 12

Typical night of sleep

Answer 12

• spend more time in slow/deep sleep earlier in the night
• spend more time in REM sleep later in the night

Question 13

Describe the activation-synthesis hypothesis of dreaming by Hobson (2004)

Answer 13

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

Question 14

Describe the coping hypothesis (clinico-anatomical hypothesis) by Valli and Revonsuo (2009)

Answer 14

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

Question 15

What is the role of adenosine in sleep?

Answer 15

Adenosine
* released by astrocytes
* signals depletion of energy
* accummulates during day after prolonged wakefulness and promotes sleep
* antagonised by caffeine

Question 16

What brain area is associated with sleep induction?

what neurotransmitters does it contain + 2 pieces of evidence

Answer 16

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

Question 17

What brain areas are associated with wakefulness and arousal?

4 areas in total + 1 piece of evidence

Answer 17

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

Question 18

Describe the flip-flop switch theory (Saper, 2001)

Answer 18

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

Question 19

What is the role of orexin in sleep?

Answer 19

• released from lateral hypothalamus
• role in maintaining arousal/wakefulness
• implicated in narcolepsy

Question 20

What are circadian rhythms?

Answer 20

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)

Question 21

Describe the experiment by Aschoff (1965)

Answer 21

• 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

Question 22

What are exogenous zeitgebers?

Answer 22

external cues that serve to set our biological clock

most important zeitgeber is light

when zeitgeber resets a biorhythm, rhythm has been ‘entrained’

Question 23

Describe how jet lag is an example of a disrupted circadian rhythm

Answer 23

mismatch between internal circadian clock and external time/cues

results in sleepiness during the day and sleeplessness at night

phase-delays vs phase-advances

Question 24

What is the brain region associated with circadian rhythms?

Answer 24

suprachiasmatic nucleus (SCN) of the hypothalamus

in experiments, lesions of the SCN disrupted circaidan rhythms of wheel running, drinking and hormonal secretion

Question 25

What did studies find about the role of the SCN?

Answer 25

• 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

Question 26

How does light reach the SCN?

and how come blind people remain entrained?

Answer 26

• 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