🎯 What is sleep?

• a reversible, behavioural state of unconsciousness where awareness of environmental stimuli is diminished

• the phenomenon of sleep is more convoluted than merely closing one’s eyelids

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💡 Why do we sleep?

• it is still a mystery as to why we spend a third of our lives asleep

• although a number of theories have been postulated:

  1. evolutionary advantage to get protection at night from wild creatures to maximise the chances of survival

  2. conservation of energy

  3. growth and repair of tissues

  4. promotion of neuroplasticity

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😴 Types of sleep

• there are two major phases of sleep:

  • non-rapid eye movement (NREM) sleep which can be subdivided into three stages

    • stage 1

    • stage 2

    • stage 3

  • rapid eye movement (REM) sleep

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🛌 One sleep cycle

• in this order:

  • wakefulness

  • NREM stage 1

  • NREM stage 2

  • NREM stage 3

  • REM sleep

• the initial cycle lasts between 70 to 100 minutes, while the subsequent cycles may last around 90 to 120 minutes

• the duration of REM sleep progressively increases in each subsequent cycle

• most individuals progress through 4 to 5 cycles per night

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Wakefulness

• EEG shows beta waves

  • although alpha waves predominate when eyes are shut

 

NREM stage 1

• EEG shows theta waves

• lightest stage of sleep

• breathing is regular and the individual may demonstrate movement

• can last up to 7 minutes

 

NREM stage 2

• EEG shows sleep spindles and K complexes (which may be associated with synaptic plasticity and memory consolidation)

• deeper sleep than stage one therefore heavy stimulation is required to wake an individual

• heart rate and body temperature drops

• lasts up to 25 minutes in the initial cycle but dominates 50% of the night

 

NREM stage 3

• EEG shows delta waves

• known as slow-wave sleep (SWS)

• deepest stage of sleep

• an individual awoken from this stage may experience “sleep inertia” which is a temporary disorientation of mental clarity

• night terrors (episodes of screaming you can’t recall) and bedwetting tend to occur in this stage

 

REM sleep

• EEG shows beta waves (similar to wakefulness)

• also known as “dream sleep” and not considered restful

• although the EEG is similar to an awake individual, all muscles are paralysed except for the extraocular muscles and the muscles that drive respiration

• takes place 90 minutes following the initiation of sleep

• the duration of REM sleep progressively prolongs in each subsequent cycle

• factors that take place during REM sleep

  • irregular breathing

  • loss of muscle tone

  • dreams

  • nightmares

  • penile and clitoral tumescence

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🧪 Neurobiology of sleep

 

​Drivers of sleep

1. circadian rhythm

   • ​under the control of the suprachiasmatic nucleus in the hypothalamus which receives input fibres from the retina via the retinohypothalamic tract conveying information about light penetrating the retina

   • the suprachiasmatic nucleus then projects to histaminergic neurons in the tuberomammillary nucleus and orexin neurons in the posterior lateral hypothalamus initiate sleep

   • the normal circadian rhythm is approximately 24.2 hours per cycle

2. sleep pressure

   • adenosine builds up in the brain during the day

   • high levels of adenosine in the brain increases the desire to sleep

 

Neural projections that maintain wakefulness

1. histamine neurons in the tuberomammillary nucleus (found in the posterior hypothalamus)

   • project to all areas of the cerebral cortex promoting wakefulness

   • inhibition of these neurons triggers sleep

2. orexin neurons (found in the posterior lateral hypothalamus)these neurons increase the activity of neurons that secrete wake-promoting neurotransmitters such as dopamine, noradrenaline, serotonin and histamine

   • the nuclei of these neurons are part of the reticular-activating system (RAS) in the brainstem which project to all areas of the cerebral cortex

 

Neural projections that promote sleep

• there are GABAergic neurons in the ventrolateral nucleus of the hypothalamus

  • these inhibit the neurons that promote wakefulness

 

Effect of caffeine on sleep

• caffeine is an adenosine receptor antagonist which blocks adenosine A2A receptors in the brain

• promotes wakefulness

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😴 Melatonin

• known as the “hormone of darkness” because it’s released at night as instructed by the suprachiasmatic nucleus

• produced by the pineal gland

• potent trigger for sleep (alerts the body it’s time for bed although it does not generate sleep)

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🚀 What's in it for you if you start prioritising sleep

• supercharges learning and memory

• promotes mental clarity - with clarity, you can make better decisions, and better decisions yield better results in terms of your personal and professional life

• revitalises your physical, mental and emotional wellbeing

• reverses your biological age and prolongs your healthspan

• strengthens your immune system

• makes you happier​​