Physiology of Sleep

• The Physiology of Sleep with a bit of breathing MEDI11-102 presented by Dr. Jacob Thorstensen, Assistant Professor of Medicine at Bond University

Objectives for today

• Describe what EEG is and identify common patterns of brain activity
• Describe consciousness clinically
• Explain the components of sleep
• Describe how the reticular formation controls wakefulness
• Outline the importance of sleep and define some common sleep disorders
• Review how the CNS controls respiration
• Explain how breathing is controlled during sleep

Why do we sleep

• Sleep is an active state of unconsciousness produced by the body
• During sleep, the brain is in a relative state of rest and is reactive primarily to internal stimuli
• The reason why we sleep is unknown

Inactivity theory

• States that being inactive at night is evolutionarily advantageous, as it reduces the likelihood of becoming prey in the dark

Energy conservation theory

• Hunting for food is less effective at night, so brain metabolism decreases during sleep to conserve calories

Restorative theory

• Body tissues are repaired during sleep with the release of growth hormones

Brain plasticity theory

• Sleep is needed for brain reorganization
• Infants sleep for more than 14 hours a day, a period of life when plasticity is at its highest

Electroencephalogram (EEG)

• Non-invasively measures the activity of the brain (cortex)
• Polysomnography (PSG), also known as a sleep study, involves HR recordings (ECG), muscle activity measures (EMG), eye muscle activity (EOG), and oxygen saturation
• Electrical activity = EEG trace
• EEG activity = summed firings of many neurons

EEG Brain State

• Awake (but somewhat focused) a Beta state (13-30 Hz) occurs, characterized by decreased amplitude and increased frequency
• Awake (but relaxed) an Alpha state (8-13 Hz) occurs
• Very deep relaxation a Theta state (4-8 Hz) occurs
• Deep sleep a Delta state (<4 Hz) occurs, characterized by increased amplitude and decreased frequency

EEG

• Absence of EEG activity (flat line) indicates no cortical activity and clinical brain death
• EEGs are used to diagnose/localize brain lesions/tumors, infarcts, sleep disorders, and epilepsy (seizures)

What is consciousness

• The state of being aware of and attentive to external stimuli
• Consciousness encompasses a spectrum from highly alert and awake to nearly asleep
• Unconsciousness ranges from a deep, unresponsive state (such as in anesthesia) to deep sleep, and even to a light dozing state
• Clinically, consciousness is defined on a continuum that evaluates behavior in response to stimuli: Alertness, drowsiness (lethargy), stupor, coma
• Level of wakefulness reflects the ongoing activity of the central nervous system (CNS)
• While asleep, one is in an unconscious state but can still be roused by normal sensory stimuli

Sleep

• Has two types, Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM)

Transitioning through sleep

• NREM sleep has four stages
• Stage 1: Start here when we begin to fall asleep
• Stages 3 and 4 equal deep sleep, also known as slow-wave sleep (SWS)
• REM is labelled as "paradoxical" sleep

Through the night

• REM gets longer in duration throughout the night
• Approximately 90 minutes to enter first REM after onset of sleep

EEG sleep stages

• Awake shows mainly beta waves
• Stage 1 shows some alpha waves
• Stage 2 shows some theta waves
• Stage 3 shows theta and delta waves
• Stage 4 shows mainly delta waves

REM vs NREM

• EEG during REM is like waking (lower amp., higher freq.) vs NREM which has higher amp., lower freq.
• Eye movement during REM has lots of activity vs NREM which has minimal activity
• HR and BP in REM varies (usually decrease compared to awake) and can increase compared to NREM while NREM decreases
• Muscle activity in REM is absent (atonia) vs NREM having some movement
• Metabolic activity in REM is like awake vs NREM having decreased metabolic activity
• Dreaming in REM is Yes vs NREM some (less than REM)
• Proportion of total sleep in REM is <25% vs NREM which is >75%

Importance of sleep

• Occurs in approximately 90 min cycles
• Periods of REM & NREM alternate throughout night
• Slow-wave sleep (stages 3/4) predominates early in sleep
• REM sleep predominates later in sleep
• There are 3 to 5 periods of REM throughout night
• Each period gets longer towards the end of sleep
• Longest dreams occur at the end of sleep (when there is lots of REM)
• Quantifying sleep and its components is not a perfect science
• Consider inter-individual differences (age, genetics, stimulant use etc.) and environmental factors (stress, sleeping conditions, temperature etc.)
• Children have more REM sleep, and very tired people have more NREM sleep compared to the "typical" sleeper

Outcomes from sleep deprived studies

• Those that slept less than 4 hrs in the 24 hr before crashing were 15 times more likely to crash their car!
• Just five nights of sleep deprivation increases risk of developing type 2 diabetes

Some sleep disorders

• Narcolepsy is characterized by excessive daytime sleepiness and one can fall asleep for 10-20 mins multiple times/day (uncontrollable)
• Insomnia is a chronic inability to obtain necessary amount of quality sleep (to function during the day) and can be caused by stress (and mental stimulation), sleeping environment, changes in sleep patterns/schedules, chemical stimulation (caffeine, drugs etc.)
• Sleep Apnea is a temporary cessation of breathing during sleep and one will wake due to hypoxia/hypercapnia, sometimes up to several hundred times a night!
• Obstructive sleep apnea is most common (made worse by obesity, alcohol)

CNS Controls Respiration

• The drive to breath is most strongly controlled by PaCO2 levels: Decreases in PaO2 < Decreases in pH < Increases in PaCO2
• High Arterial Pco2 leads to low pH in brain extracellular fluid (ECF)
• Central chemoreceptors in the medulla respond to H+ in brain ECF, mediating 70% of the CO2 response
• Peripheral chemoreceptors in carotid and aortic bodies mediate 30% of the CO2 response
• Afferent impulses are sent to the medullary respiratory centers
• The respiratory muscles increase ventilation and exhale more CO2, returning arterial Pco2 and pH to normal
• Sensitivity to PaCO2 is reduced during sleep
• Low PaO2 can also amplify response to PaCO2

Breathing controls during Sleep Versus Wake

• Compared to Waking during REM ventilation is slight and irregular and airway resistance is increased while during NREM ventilation is decreased but more regular and the airway resistance is increased
• During REM and NREM PaO2 is slightly altered
• During REM and NREM PaCO2 is slightly incrased
• During REM and NREM the response to decreased PaO2 is decreases
• During REM and NREM the response to increased PaCO2 is decreases

Obstructive sleep apnoea

• It causes signifiant breathing pauses during sleep because of airway obstruction
• Airway collapse leads to breathing cessation
• Hypoxia/hypercapnia triggers awakenings from deep sleep to restore breathing
• Frequent wake ups are needed to resume breathing
• Severe levels of apnoea can require the aid of a CPAP (Continuous Positive Airway Pressure) to maintain pressure and keep the airway open