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NORMAL SLEEP ARCHITECTURE

Sleep architecture results from the recurrence of different states of activity of the brain, and may be described as it results from analysis of: • electroencephalogram (EEG), for detection of cortical brain activity; • electrooculogram (EOG), for detection of eye movements; • electromyogram (EMG) of submental muscles, for detection of postural tone

SLEEP STRUCTURE (1) Analysis by “epochs” Analysis on consecutive periods (“epochs”) lasting 20 or 30 seconds, that are scored according to standard rules (or rules of Rechtschaffen and Kales) (Rechtschaffen A, Kales A. Los Angeles, CA: BIS/BRI, UCLA, 1968)

SLEEP STRUCTURE (2) A more complete evaluation of sleep structure includes also analyses based on events lasting less than scoring epochs: •

Arousals (Sleep 1992;15:174-84)

•

Cyclical alternating pattern or CAP

(Sleep Med 2001;2:537-53)

MINIMAL REQUIREMENTS FOR SLEEP MONITORING (1) 1. EEG If only one channel for EEG is available a central unipolar derivation is required (C3A2 or C4A1) If an additional channel is available an occipital derivation is recommended (O2A1 or O1A2)

MINIMAL REQUIREMENTS FOR SLEEP MONITORING (2) 2. EOG

3. EMG

Two EOG recordings are recommended. Electrodes are placed on right and left outer cantus (ROC and LOC, respectively) and referred to ipsilateral or contralateral mastoid (A1 or A2)

One EMG is recorded bipolarly by two electrodes placed on mentalis/submentalis muscles

EEG RHYTHMS AND WAVES • Alpha rhythm: 8-13 cycles/s • Theta rhythm: 4-7 cycles/s • Delta rhythm: < 4 cycles/s • Vertex sharp wave: sharp transient, 50-200 ms long, up to about 200 µV high • K complex: waveform consisting of a rapid negative (upwards) component followed by a slower positive component, and lasting 0.5-2 s • Spindles: sequence of waves at 12-14 cycles/s, lasting 0.5-1.5 s

STAGES ACCORDING TO RECHTSCHAFFEN AND KALES • wakefulness • • • •

stage 1 stage 2 stage 3 stage 4

• REM

Stages 1-4 = non-REM (NREM) sleep Increasing sleep depth from stage 1 to 4

REM = rapid eye movement

RELAXED WAKEFULNESS • EEG: low voltage mixed frequency (when eyes open) or alpha activity (only when eyes are closed) • EOG: possible rapid eye movements, or slow rolling eye movements announcing stage 1 • EMG: usually, high tonic activity

alpha waves rolling eye movements 2.5 s

STAGE 1 • EEG: relatively low voltage mixed frequency activity (prominent theta); possible vertex sharp waves • EOG: possible rolling eye movements • EMG: trend to a decrease in tonic activity vertex sharp wave

2.5 s

STAGE 2 • EEG: relatively low voltage mixed frequency activity plus K complexes and spindles • EOG: no eye movements • EMG: usually, low tonic activity

2.5 s

K complex

spindles

STAGE 3 • EEG: >20% but <50% delta waves, >75 µV each • EOG: no eye movements • EMG: usually, low tonic activity

2.5 s

STAGE 4 • EEG: >50% delta waves >75 µV each • EOG: no eye movements • EMG: usually, low tonic activity

2.5 s

STAGE REM • EEG: relatively low voltage, mixed frequency activity; possible some sawtooth waves and alpha • EOG: rapid eye movements • EMG: no tonic activity

2.5 s

rapid eye movements

NORMAL SLEEP (1) • NREM sleep is the first to appear • NREM sleep progresses gradually from lighter to deeper stages • First REM stage appears 70-90 minutes after sleep onset • Sleep is organized in “cycles”

SLEEP CYCLE

A whole REM stage plus the NREM sleep time preceding that stage Approximate duration in adults: 90 minutes

NORMAL SLEEP STRUCTURE (2)

Sequence of 4 to 6 sleep cycles in a night Stages 3 and 4 become progressively shorter, and REM sleep progressively longer, from the first to the last sleep cycle

NORMAL STAGES DURATIONS (% OF TOTAL SLEEP PERIOD) DURING A NIGHT IN YOUNG ADULTS

• Stage 1: 2-5% • Stage 3: 5-8% • Stage REM: 2025%

• Stage 2: 45-55% • Stage 4: 10-15% • Wakefulness: <5%

EXAMPLE OF NORMAL SLEEP IN A YOUNG ADULT

NORMAL SLEEP AND AGE • Short sleep cycles are already present at birth • The four NREM stages may be identified at about the 6th month of life • REM duration decreases from 50% at birth to 20-25% approximately in the 5th year of life and then stabilizes • Stages 3 and 4 progressively decline after the age of 20 and may even disappear in the elderly • Stage 1 and wake in the sleep period increase in old age

AROUSALS Phasic events, causing sleep fragmentation They may or may not result in full behavioural awakening They may be followed by a shift to a lighter sleep stage Variably identified. Most widely used criteria to score arousals: ASDA rules (1992)

MAIN ASDA RULES TO SCORE AROUSALS • Abrupt shift towards faster EEG rhythms, which may include theta, alpha or frequencies >16/s, but not spindles • Scoring independent of Rechtschaffen and Kales staging • Preceded by at least 10 seconds of uninterrupted sleep • Minimal duration: 3 seconds • Increase in EMG amplitude possible in NREM but necessary in REM sleep

EXAMPLE OF AROUSAL IN NREM SLEEP

EXAMPLE OF AROUSAL IN REM SLEEP

2.5 s

NORMAL NUMBER OF AROUSALS DURING SLEEP Rate of occurrence of arousals is highest in stage 1, and lowest in stages 3 and 4 Number of arousals/hour of sleep time is called “arousal index” and increases with age Arousal index has a wide normal range Mean value in teenagers: about 10/h Mean value in elderly: about 25/h

CYCLIC ALTERNATING PATTERN (CAP) Periodic activity of NREM sleep, represented by cycles including two phases, lasting 2-60 seconds each: • Phase A: sudden change in frequency and amplitude of EEG waves, that may include various phenomena like ASDA arousals or slow high voltage rhythms (delta bursts, K complex sequences) • Phase B: background tonic activity of the sleep stage

NREM sleep may show CAP or non-CAP features • CAP: at least two consecutive cycles (i.e. two phases A and two phases B) • Non-CAP: absence of CAP for >60 seconds

% of NREM sleep with CAP: CAP rate

EXAMPLES OF CAP AND NON-CAP

Reprinted from : Sleep Medicine Reviews 2000;4:101-23, with permission from Elsevier

CAP: NORMAL RATE AND SIGNIFICANCE Normal CAP rate changes with age (from about 43% in teenagers, to 32% in young adults, to 55% in elderly) CAP is considered a marker of sleep instability and correlates with subjective sleep quality

COMMONLY USED ACRONYMS AND DEFINITIONS (1) • TIB = • TST =

time in bed

• TSP = SPT =

total sleep period, or sleep period time: time from beginning of the first to the end of the last sleep epoch

total sleep time

• WASO = wake after sleep onset: total duration of wake epochs in the SPT • SWS = slow wave sleep: generally used as synonymous of stages 3 and 4

COMMONLY USED ACRONYMS AND DEFINITIONS (2) • Sleep latency: time from lights out to sleep onset • Sleep onset: usually defined as beginning of first of at least three consecutive sleep epochs or of any first single sleep epoch different from stage 1 • REM latency: time from sleep onset to beginning of first REM epoch (minus intermediate wakefulness) • Sleep efficiency: TST/TIB*100