Sleep stages

Question 1

What happens when Awake with Eyes Open?

Answer 1

Eyes open:

Low voltage, mixed frequency (beta)

Dominant frequency: 13–30 Hz (beta rhythm), especially frontal

Lots of eye blink artifacts frontally (slow upward deflections)

Question 2

What happens when Awake with Eyes Closed?

Answer 2

Eyes closed:

Appearance of posterior dominant rhythm (PDR) — also called alpha rhythm

8–12 Hz, best seen over occipital regions (O1, O2)

Alpha attenuates or “blocks” with eye opening (alpha reactivity)

Question 3

What happens in Drowsiness or Stage 1 sleep/N1?

Answer 3

Transition from wakefulness

Background becomes slower:

• Alpha rhythm fades
• Theta waves (4–7 Hz) begin to dominate
• Slow rolling eye movements
• May see vertex sharp waves — more common in children
• No spindles or K-complexes yet

Question 4

What happens in Stage 2 sleep/N2?

Answer 4

Defined by appearance of:

Sleep spindles (11–16 Hz bursts, central leads)

K-complexes (sharp negative-positive biphasic waveform, frontal-central)

Background is mainly theta

Eye movements stop

This is usually the longest stage of NREM sleep in a full cycle

Question 5

What happens in Stage 3 sleep/N3? Also called deep sleep or slow wave sleep?

Answer 5

Dominated by high-amplitude delta waves (0.5–2 Hz)

Must occupy at least 20% of the epoch

Often seen frontally

Also called slow wave sleep

Sleep spindles and K-complexes may still be present, but delta dominates

Question 6

What happens in REM sleep?

Answer 6

EEG looks like wakefulness:

Low voltage, mixed frequency (theta, some beta)

No spindles or K-complexes

Rapid eye movements visible on EOG

Reduced chin EMG tone (in full polysomnography)

No prominent slow waves

May see sawtooth waves (not always)

Question 7

What is the sleep architecture seen on EEG with age?

Answer 7

Sleep architecture varies with age:

Newborns may show trace alternant or trace discontinu instead of adult-like stages

Sleep spindles and K-complexes appear around 2–3 months

Vertex sharp waves and spindles are more prominent in children

PDR is slower in younger children (e.g. 6–8 Hz at 4 years old)

Question 8

Wakefulness and REM sleep can look surprisingly similar on EEG, because both show low-voltage, mixed-frequency activity. How do you tell them apart?

Answer 8

EEG similarities: Low-voltage, mixed-frequency activity (mostly theta and some beta). They both look “desynchronized” — not slow-wave

Differences:

1. Eye Movements:
   During wakefulness, if the eyes are open, you often see eye blinks as large slow artifacts frontally. When the eyes are closed, eye movements stop and an alpha rhythm appears. In contrast, REM sleep is characterized by rapid eye movements, which can be seen as sharp deflections on the EOG channels (if recorded), even though the EEG background looks awake-like.
   
   2. Muscle Tone:
      In wakefulness, muscle tone is normal. If you have EMG channels, there will be consistent baseline activity. In REM sleep, there is generalized muscle atonia — meaning the muscle tone is greatly reduced. This is evident as a flat EMG tracing in polysomnography.
   3. Alpha Rhythm:
      An alpha rhythm (8–12 Hz) appears over the occipital region during relaxed wakefulness with eyes closed, and it blocks (disappears) with eye opening. In REM sleep, alpha is typically absent, and the EEG consists instead of low-voltage, mixed-frequency activity.
   4. Sawtooth Waves:
      These are seen only in REM sleep, not in wakefulness. They are not always present, but when they are, they appear as 2–6 Hz sharply contoured waves over frontal-central leads.
   5. Stage Progression:
      Wakefulness comes before sleep onset and drowsiness (Stage 1). REM sleep only occurs after progression through NREM sleep, particularly after Stage 2 and Stage 3. It follows a predictable cycling pattern throughout the night.
   6. Reactivity:
      Wake EEG is reactive — alpha rhythm will block with eye opening or sensory stimuli. REM EEG is not reactive, because the brain is in a dissociated state where external stimuli don’t cause typical EEG changes.

Question 9

Wakefulness vs REM snapshot

Answer 9

If you just have EEG (no EMG or EOG), REM sleep:
• Lacks alpha rhythm
• Occurs after prior sleep stages
• Has a more diffuse, irregular theta-dominant pattern
• No reactivity to noise or stimuli

In contrast, wakefulness:
• Shows alpha rhythm with eyes closed
• Is reactive (e.g. alpha blocks with eye opening or sound)
• Shows voluntary or blink-related movement artifacts