Sleep and Circadian Rhythms

Sleep

A natural, periodic state that involves reduced responses to environmental stimuli and decreased mobility

We cycle through each of the three stages and REM, with each cycle lasting approximately 90 minutes

Homeostatic (S) - if we do not sleep we accumulate sleep debt

Circadian (C) – sleep tends to happen at a particular time during the 24-hour cycle

Coma

An extended period of unconsciousness

Brain death

No sign of brain activity and no response to stimuli

Polysomnography

The 'gold standard' of sleep research that involves recordings of electrical activity from multiple sources.

EEG (electroencephalogram)

Recordings of activity of populations of neurons in the brain underneath the skull.

EOG (electrooculogram)

Recordings of activity of the muscles around the eyes to decipher eye movements.

EMG (electromyogram)

Recordings of the activity of the muscles in the body.

Beta Waves

Irregular activity of 13–30 Hz observed during wakefulness when the brain is processing information.

Alpha Waves

Activity of 8–12 Hz observed when a person is resting quietly.

Stage 1 Sleep

Theta activity (3.5–7.5 Hz) - transition between sleep and wakefulness

Stage 2 Sleep

Sleep begins - irregular activity and also sleep spindles (12-14Hz) (although these also occur in other stages of sleep) and K complexes

Stage 3 Sleep

High-amplitude and low-frequency delta activity (less than 3.5 Hz)

Slowing down of brain activity as well as other bodily functions, such as heart rate, breathing, temperature, kidney function, etc

Sometimes referred to as slow-wave sleep (SWS), or deep sleep

REM Sleep

A sleep phase characterized by increased brain activity and asynchrony in brain waves accompanied by muscle atonia

Dreams

Discovered by Dement and Kleitman, 1957 when participants were awakened from REM sleep they tended to report vivid dreams

Freud - dreams are the ‘royal route to the unconscious’, secret wishes

Jung - dreams are a glimpse into the collective unconscious

Paradoxical sleep

Deep sleep, in terms of muscle activity but light sleep, in terms of brain activity

Activation Synthesis Hypothesis of Dreams

Hobson (2004) - no meaning in dreaming, even though dreams are based on each person’s experiences

The brainstem is activated during REM and sends signals to the cortex (visual association cortex) which creates images with actions and emotions

The frontal cortex is less activated during dreaming so there is no logic in the timing or the sequence of events

The person tries to organise the content into a logical story when awake, so it is a bottom-up view on dreams

The Neural Basis of Sleep

Many natural substances cause sleep i.e. morphine (Morpheus)

Neurochemicals and hormones can produce sleep - wake cycles

CSF (Cerebrospinal Fluid) Transfer Experiments

CSF from sleep-deprived animals, when transferred to well-rested animals, induced sleep—suggesting the presence of sleep-promoting substances (somnogens) in the CSF, such as adenosine and immune molecules. This supported the idea that sleep is regulated by chemical build-up during wakefulness.

Adenosine

Accumulates during the day, after prolonged wakefulness and promotes sleep

Ventro-lateral preoptic area (vlPOA)

Area of the anterior hypothalamus containing inhibitory neurotransmitters such as GABA; damage to this area causes insomnia.

Reticular Activating System-RAS

The reticular formation (Reticular Activating System-RAS) comprised by a group of nuclei in the brainstem that send projections to the forebrain to promote arousal

Is responsible for wakefulness.

Flip-flop switch (Saper, 2001)

The flip-flop switch model proposes that sleep and wake states are controlled by mutually inhibitory circuits—wake-promoting regions (like the RAS) inhibit sleep centres, and vice versa—allowing for rapid and stable transitions between sleep and wakefulness

Functions of sleep

Conservation of energy - during NREM body temperature drops and muscle activity decreases

Restoration - restorative mechanisms take place to remove the free radicals and toxic waste

Promotes development - during stage 3 sleep (SWS), Growth Hormone (GH) release is at its peak which is important for growth

Facilitates cognition - sleep improves memory and learning by helping the brain consolidate information

Circadian rhythms

Rhythms or regular patterns of activity associated with a 24h-cycle such as day and night.

Endogenous cues vs exogenous zeitgebers

Orexin or Hypocretin

A peptide released from the lateral hypothalamus (LH) highly responsible for the maintenance of wakefulness and implicated in narcolepsy

Zeitgebers

External cues that serve to set our biological clock

Jet Lag

A mismatch of the internal circadian clock and external time.

Suprachiasmatic Nucleus (SCN)

The primary biological clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus.

Retinohypothalamic Tract

The SCN receives information about light through this tract, formed by a special population of ganglion cells (photosensitive retinal ganglion cells-PRGCs)

PRGCs have their own photopigment called melanopsin and can respond directly to light, especially blue light (Provencio et al 2000)

SCN and other brain areas

The SCN regulates behaviour, including waking and sleeping, by controlling activity levels in other brain areas and secretory glands such as the pituitary and the pineal gland

Sleep Hygiene

Principles that can help work in-sync with our circadian rhythms to promote better sleep