Lecture 9: Sleep, Dreaming and Circadian Rhythms

recuperation theories of sleep

-sleep is needed to restore homeostasis

-wakefulness causes a deviation from homeostasis

adaption theories of sleep

-sleep is the result of an internal timing mechanism

-evolved to protect us from the dangers of the night

comparative analysis of sleep

-all mammals and birds sleep

-not a special higher-order human function

-not necessarily needed in large quantities

-no clear relationship between species’ sleep time and activity level

internal timing mechanism

-results in sleep

-sleep and wakefulness follow circadian rhythm with periodicity of about 24 hours

-circadian rhythms are endogenous and persist without environmental cues

-modulated by external timing cues → adapt rhythm to the environment

hypothalamus

-controls body temperature, hunger, thirst and circadian cycles

suprachiasmatic nucleus (SCN)

-major internal clock of the medial hypothalamus

-retinohypothalamic tract sends light signals from the environment to the hypothalamus/SCN

-lesioning of this tract dampens down the circadian rhythm of sleep

-SCN regulates timing of sleep → not responsible for sleep

sleep deprivation studies with laboratory animals

-carousal apparatus used to deprive rats of sleep

• when experimental rat’s EEG indicates sleep, the chamber floor moves → if rat does not wake then it falls into water

-experimental rats typically die after several days

-post-mortem studies reveal extreme stress

-difficult to separate effects of stressors used to prevent sleep from the effects of lost sleep

sleep

-defined behaviourally as:

• reduced motor activity

• reduced response to stimulation

• stereotypic postures

• relatively easy reversibility

psychological activity of sleep

-measured using electrical recordings:

• muscle movements with electromyography

• eye movements with electro-oculography

• brain activity with EEG

frequency

-EEG waves → measured brain activity during sleep

-cycle is peak to peak

• low frequency waves → slow, more time between peaks

• high frequency waves → fast, less time between peaks

alert (EEG stages of sleep)

-high frequency, low amplitude activity

-beta activity

relaxed (EEG stages of sleep)

-medium frequency, medium amplitude activity

-alpha activity

stage 1 (EEG stages of sleep)

-transition between sleep and wakefulness

-muscles are still active, eyes show slow and gentle rolling movements

-some theta activity

stages 2 and 3 (EEG stages of sleep)

-sleep gets deeper and deeper

-theta activity in stage 2

-stage 3 mostly delta activity

-EEG gets progressively lower in frequency and higher in amplitude

stage 4 (EEG stages of sleep)

-deepest stage of sleep

-reached in less than an hour and continues for up to half an hour

-high amplitude

-delta activity

REM sleep (EEG stages of sleep)

-EEG looks like that of a person that is awake and active

-EMG generally quiet

pons

-has a clock that is responsible for the basic rest-activity cycle (BRAC) and cycles of REM sleep and slow-wave sleep

reticular formation

-set of interconnected nuclei located throughout the brainstem

pontine reticular formation

-part of pontine reticular formation

-brain region without clearly defined borders in the centre of the pons

evidence that reticular activating system is involved in sleep

1. cats with midcollicular transection displayed a pattern of continuous slow-wave sleep in their EEGs

2. lesions at the midcollicular level that damaged the core of the reticular formation produced EEG indicative of continuous slow-wave sleep

3. electrical stimulation of the pontine reticular formation desynchronised the EEG and awakened sleeping cats

4. cats with transection of caudal brain stem displayed a normal sleep-wake cycle

little effect of sleep deprivation

-logical deduction, critical thinking

-physical strength and motor performance

larger effect of sleep deprivation

-executive function (prefrontal cortex)

-assimilating changing information

-updating plans and strategies

-innovative, lateral, insightful thinking

-reference memory

3-4 hours of deprivation in one night (experimental studies of sleep deprivation in humans)

-increased sleepiness

-disturbances displayed on written tests of mood

-poor performance on tests of vigilance

2-3 days of continuous deprivation (experimental studies of sleep deprivation in humans)

-experience microsleeps → naps of 2-3 seconds

sleep deprivation increases sleep efficiency

-after deprivation, most of lost stage 4 is regained and SWS is increased

-short sleepers get as much SWS as long sleepers

-naps without SWS do not decrease the night’s sleep

-gradual reductions in sleep time lead to decreases in stages 1 and 2

-little sleepiness produced with repeated REM awakenings, unlike SWS

REM sleep deprivation

-deliberately wake people when in REM, then allow them to continue sleep

-two consistent effects:

• proceed more rapidly into REM as REM deprivation increases

• REM rebound → more time spent in REM when deprivation is over

-suggests that REM sleep serves a special function

purpose of REM

-processing of explicit memories → inconsistent findings and REM-blocking drugs do not interfere with memory

-alternative explanation is it is difficult to remain in non-REM sleep

• sleepers woke from REM for 15 minutes were awake for a bit rather than REM and found no sleepiness or REM rebound the next day

• REM-blocking drugs cause periods of wakefulness

REM development

• 70% REM new borns

• 30% REM 6 months

• 22% REM adulthood

• 15% REM late adulthood

REM sleep and dreaming

-80% of awakenings from REM yield reports of story-like dreams

-external stimuli may be incorporated into dreams

-sleepwalking and talking are less likely to occur while dreaming

-non-REM dreams are isolated experiences such as falling

Freud’s explanation (why we dream)

-dreams are triggered by unacceptable repressed wishes

• manifest dreams → what we experience

• latent dreams → the underlying meaning

activation-synthesis theory (why we dream)

-dreams are due to cortex’s attempt to make sense of random brain activity

depression and sleep

-depression is associated with both insomnia and hypersomnia

-total sleep deprivation for one whole night improves depressive symptoms in 40-60% of treatments

Alzheimer’s and sleep

-sleep facilitates waste clearance from the brain

-cerebrospinal fluid contains amyloid which can lead to build up of plaques and is linked to Alzheimer’s → sleep facilitates the waste clearance of this fluid