Sleep, Dreams, and Circadian Rhythms - Lecture Notes

Week Seven Lecture: Sleep, Dreams, and Circadian Rhythms

Introduction

This week's lecture focuses on sleep, dreams, and circadian rhythms. The content will cover the definition and characterization of sleep, dream theories, the timing of sleep, and the effects of inadequate sleep.

Acknowledgment of Country

Lecturer pays respects to the Boon Wurrung people of the Kulin Nations, the traditional owners of the land where the Monash Sleep and Circadian Medicine Lab is located. Encourages everyone to reflect on the traditional owners of the lands they are on.

Part 1: What is Sleep and Why Do We Dream?

Defining and Measuring Sleep

Polysomnography (Sleep Study):
- Monitors electrophysiological parameters during sleep.
- Conducted at night in clinical or research settings.
- Involves attaching electrodes to the scalp, areas around the eyes, and typically the chin.
- Chin electrodes measure muscle tension.
- Additional electrodes may be placed on the legs.
Clinical Measures in Polysomnography:
- Pulse oximetry: Measures blood oxygen saturation.
- Airflow: Measures air pressure from the nose via a cannula.
- Microphone: Detects snoring.
- Belts: Monitor breathing effort by measuring chest movement.
Key Measures for Defining Sleep Stages
- EEG (Electroencephalogram): Electrical activity of the brain.
- EOG (Electrooculogram): Electrical activity of muscles around the eyes.
- EMG (Electromyogram): Electrical activity of muscles
Historical Understanding of Sleep:
- Initially thought to be a passive process where the brain shuts off.
- Now known to be a dynamic process with different brain activity periods.
Brain Activity Measurement:
- Measured by amplitude (height of the signal) and frequency (cycles per second).
- Frequency is related to sleepiness: faster frequency indicates alertness, slower frequency indicates drowsiness or deeper sleep.
- Activity measured in microvolts (very small measure of electrical activity).
**EEG and Neuronal Activity: **
- Electrode on the scalp picks up activity from a population of neurons, not a single neuron.
Sleep Recording Analysis:
- Recordings are divided into 30-second segments called epochs.
- Each epoch is assigned a sleep stage based on EEG, EOG, and EMG characteristics.

Wakefulness

EEG Activity:
- Beta activity (12 to 30 hertz) during vigilant wakefulness.
- Desynchronized: Neurons firing at different times, resulting in high frequency, low amplitude signal.
Eye and Muscle Activity:
- High activity in eye muscles (blinking, reading) and general muscle tension.

Stages of Sleep

NREM (Non-Rapid Eye Movement) Sleep:
- Classified into stages N1, N2, and N3.
- NREM Stages: N1, N2, and N3 get deeper as the number increases.
NREM Stage 1:
- First stage of sleep, lasts for 2-5% of total sleep time.
- Characterized by low amplitude, mixed-frequency EEG with predominantly theta activity (4-7 hertz).
- May include sharp vertex waves.
- Slow, sinusoidal eye movements (eyes rolling back in the head).
- Slight reduction in muscle tension.
NREM Stage 2:
- Similar EEG activity to stage N1 (low amplitude, mixed frequency, predominantly theta).
- Unique EEG events: sleep spindles (bursts of 11-16 hertz activity, typically 12-14 hertz lasting about 0.5 second) and K complexes (negative sharp wave followed by a positive component, lasting about 0.5 second).
- Eyes are settled, with no significant eye movements.
- Tonic muscle activity (baseline muscle tension) is present, lower than wakefulness.
NREM Stage 3 (Slow Wave Sleep):
- Deepest stage of sleep; most difficult to wake someone.
- Characterized by high amplitude, low-frequency EEG activity (0.5-2 hertz) called delta activity.
- Requires a minimum amplitude of 75 microvolts.
- No eye movements.
- Lower level of tonic muscle activity.
- Sleep inertia: Feeling groggy when woken from this stage.
REM (Rapid Eye Movement) Sleep:
- Brain activity is low amplitude, mixed frequency EEG pattern similar to wakefulness or NREM stage 1.
- Bursts of rapid eye movements (phasic REM activity).
- Muscle atonia: Complete loss of muscle tone due to inhibition of motor activity.
- Amygdala is active, suggesting processing of emotional dreams.
- Important for brain development, with greater amounts in younger years.
- Sawtooth waves may be present in the EEG.

Summary of Sleep Stages

NREM Stage 1:
- Low amplitude, mixed-frequency activity.
- Lightest stage of NREM sleep.
NREM Stages 2 and 3:
- Increase in depth.
- Theta activity in stages 1 and 2; delta activity in stage 3.
- Low tonic muscle tone.
- Minimal eye movement (slow eye movements in stage 1).
REM Sleep:
- Rapid eye movements (tonic-phasic REM).
- Lowest muscle tone (muscle atonia).

Progression Through Sleep Stages and Sleep Cycles

Typical Sleep Progression:
- Starts in NREM stage 1, progresses to stage 2, then stage 3, and back up to stages 2 and 1 before entering REM sleep.
Sleep Cycle:
- Progression through various stages of sleep (NREM 1-2-3-2-1-REM).
- Each cycle lasts about 90 minutes.
- Sequential cycles occur throughout the night (5-6 cycles typically).
Changes in Sleep Cycles Across the Night:
- The composition of sleep stages changes across the night.
- The first third of the night has more slow-wave (NREM stage 3) sleep, while the latter half has more REM sleep.

Dreaming and REM Sleep

Initial Understanding:
- Early research suggested REM sleep was the primary stage for dreaming.
Current Understanding:
- Dreaming occurs in both REM and NREM sleep.
- REM dreams are often more emotional due to amygdala activation.
- Antidepressants can reduce REM sleep without affecting dream recall.
- Cortical lesions that abolish REM sleep do not eliminate dreaming.

Theories of Why We Dream

Activation Synthesis Theory (Hobson):
- Dreams result from random signals sent from the brainstem to the cortex.
- The brain attempts to make sense of these random signals.
Evolutionary Theory (Revonsuo):
- Dreams serve an evolutionary purpose by preparing us for threatening events.
- Practicing threatening events in dreams can improve preparedness in real life.
Proto-consciousness Hypothesis (Hobson, revised):
- Dreams serve a purpose and provide an advantage by stimulating various scenarios, not just threatening ones.
- Important for early development when sensory input is limited.
- Dreams help anticipate and predict how events will unfold while awake.
- Dreams are a training mechanism representing real-life scenarios.

Brain Regions Involved in Dreaming

Based on lesion and brain imaging studies (primarily in REM sleep).
Increased activity in the temporoparietal junction, medial prefrontal cortex, and medial occipital cortex during REM sleep.
The medial occipital lobe is associated with visual imagery in dreams.