Principles of Sleep

Sleep is a naturally recurring state of mind and body, characterized by altered consciousness, relatively inhibited sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings.

Basics

Stages of Sleep with Polysomnograph Recordings Figure 1 Sleep consists of two physiological states: non-rapid eye movement sleep (known as NREM) and rapid eye movement sleep (also known as REM):

  1. NREM sleep can be broken down into three stages: N1, N2, and N3 (or four stages based on: Stages 1, 2, 3/4).
  2. REM sleep is a different state of sleep characterized by high levels of brain activity and physiological activity similar to an awake state. REM sleep usually develops about 90 minutes after sleep begins. REM sleep on a polysomnograph appears the same as the awake stage. Therefore, REM cannot be identified by EEG alone, you need an EMG (and see absence of muscle activity) to confirm the presence of REM.

Sleep is also posited to play a role in the immune system.[1]

Physiologically Drowsy States

Certain periods of the day also contribute to sleepiness and drowsiness. The afternoons (12pm to 1pm), and early evenings (4pm to 5pm) are physiologically drowsy states when people feel more sleepy. This is evidenced in certain cultures, like afternoon siestas in Spain.[2]

Sleep Stages

Awake
Description Wakeful state
EEG Alpha waves (high frequency, 8 to 12 Hz)
Eye Movements Eyes move and blink
Muscle Tone High voluntary tonic activity
Stage N1 (Stage 1)
Description Transition from wakefulness to sleep. Marker for very light quality sleep or sleep disruption. Occupies 5% of sleep cycle. Patients may often be in this stage, and think they are “not sleeping.”
EEG Alpha waves (< 50% total), mixed with Theta waves (slow waves, 3-7 Hz)
Eye Movements Slow eye movements
Muscle Tone Decreasing levels of high tonic activity
Stage N2 (Stage 2)
Description Occupies 50% of sleep cycle. Sleep spindles and K-complexes emerge during this stage. They are thought to be the brain's way of evaluating potential threats (i.e. external stimuli) while sleeping and to dampen arousals if the threats are not real.
EEG Sigma waves (also known as sleep spindles, 11-16 Hz) with K-complexes (negative sharp wave followed by positive slow waves, 12-14 Hz)
Eye Movements None
Muscle Tone Low tonic activity
Stage N3 (Stage 3 and 4)
Description Occupies 10-20% of the sleep cycle. This is homeostatic sleep (reduced BP, HR, cardiac output, RR), growth hormone is released.
EEG Delta waves (low frequency, <2 Hz, with high voltage, >75 μV).
Eye Movements None
Muscle Tone Low tonic activity
Rapid Eye Movement (REM)
Description “Paradoxical state.” Resembles awake state, except there is muscle paralysis. Arousal and increased variability of autonomic state (BP, HR). Increased brain temperature, cerebral glucose metabolism and cerebral blood flow.
EEG Low-voltage, mixed frequency activity, including: saw-tooth waves, theta activity, and slow alpha
activity.
Eye Movements Rapid eye movement
Muscle Tone Almost total muscle paralysis
Stage of Sleep Description EEG Eye Movements Muscle Tone
Awake Wakeful state Alpha waves (high frequency, 8 to 12 Hz) Eyes move and blink High voluntary tonic activity
Stage N1 (Stage 1) Transition from wakefulness to sleep. Marker for very light quality sleep or sleep disruption. Occupies 5% of sleep cycle. Patients may often be in this stage, and think they are “not sleeping.” Alpha waves (< 50% total), mixed with Theta waves (slow waves, 3-7 Hz) Slow eye movements Decreasing levels of high tonic activity
Stage N2 (Stage 2) Occupies 50% of sleep cycle. Sleep spindles and K-complexes emerge during this stage. They are thought to be the brain's way of evaluating potential threats (i.e. external stimuli) while sleeping and to dampen arousals if the threats are not real. Sigma waves (also known as sleep spindles, 11-16 Hz) with K-complexes (negative sharp wave followed by positive slow waves, 12-14 Hz) None Low tonic activity
Stage N3 (Stage 3 and 4) Occupies 10-20% of the sleep cycle. This is homeostatic sleep (reduced BP, HR, cardiac output, RR), growth hormone is released. Delta waves (low frequency, <2 Hz, with high voltage, >75 μV). None Low tonic activity
Rapid Eye Movement (REM) “Paradoxical state.” Resembles awake state, except there is muscle paralysis. Arousal and increased variability of autonomic state (BP, HR). Increased brain temperature, cerebral glucose metabolism and cerebral blood flow. Low-voltage, mixed frequency activity, including: saw-tooth waves, theta activity, and slow alpha
activity.
Rapid eye movement Almost total muscle paralysis

Measuring sleep quality can be done through scales, questionnaires, and sleep diaries:

Measurement Scales for Sleep Quality

Epworth Sleepiness Scale (ESS)
Rater Patient
Description The ESS is a self-administered questionnaire with 8 questions.
Download ESS Information
Insomnia Severity Index (ISI)
Rater Patient
Description The ISI has seven questions rated on a scale of 0 to 4.[3]
Download ISI Download
Pittsburgh Sleep Quality Index (PSQI)
Rater Patient
Description The PSQI is an effective instrument used to measure the quality and patterns of sleep in adults. It differentiates “poor” from “good” sleep quality by measuring 7 components.
Download PSQI Download
Name Rater Description Download
Epworth Sleepiness Scale (ESS) Patient The ESS is a self-administered questionnaire with 8 questions. ESS Information
Insomnia Severity Index (ISI) Patient The ISI has seven questions rated on a scale of 0 to 4.[4] ISI Download
Pittsburgh Sleep Quality Index (PSQI) Patient The PSQI is an effective instrument used to measure the quality and patterns of sleep in adults. It differentiates “poor” from “good” sleep quality by measuring 7 components. PSQI Download
See main article: Sleep Disorders

Sleep-wake disorders encompass 10 disorders or disorder groups in the DSM-5, as listed below. Individuals with these disorders typically present with sleep-wake complaints of dissatisfaction regarding the quality, timing, and amount of sleep. Resulting daytime distress and impairment are core features shared by all of these sleep-wake disorders.

  • When Stage 1 is high = patient is constantly having light sleep
  • Waking up during the night is not uncommon or pathological; the time it takes to fall back asleep is what affects sleep quality

Adequate sleep is vital for good mental health. Evidence is mounting that poor sleep or insomnia can contribute to worsen of psychotic symptoms, including hallucinations.[5]

References