Sleep Notes

The upcoming topic will cover sleep, circadian rhythms, and sleep disorders.
The lecture aims to define sleep scientifically, explore brain mechanisms controlling sleep, and discuss sleep disorders.

Sleep is studied using polysomnography, which involves recording multiple physiological measures.
Polysomnogram includes:
- EEG (Electroencephalogram): Brain activity
- EOG (Electrooculogram): Eye movements
- EMG (Electromyography): Muscle activity
- EKG (Electrocardiogram): Heart activity
- Airflow and blood oximetry: Oxygen levels
Sleep studies involve individuals sleeping in a sleep clinic while connected to these monitoring devices which may affect results.
Sleep is a common phenomenon in the animal kingdom, observed in vertebrates and increasingly in invertebrates like spiders and insects.

Changes in EEG (brain activity) are the defining characteristics of sleep stages.
EEG measures electrical activity from the brain using electrodes on the scalp.
EEG electrodes record the summation of membrane potentials from large groups of neurons in the cortex.
The recorded signal depends on whether neurons are synchronized or acting independently.
Synchronized neuronal activity produces larger deviations (changes in voltage) in the EEG signal.

EEG analysis focuses on patterns of synchrony and the speed of changes, expressed as wave frequencies.
Wave frequency is measured in hertz (Hz), indicating the number of changes per second.
- $f = \frac{\text{number of cycles}}{\text{time in seconds}}$
EEG waves are classified into frequency bands:
- Beta waves: 13-30 Hz
- Alpha waves: 8-13 Hz
- Theta waves: 3.5-7.5 Hz
- Delta waves: Under 4 Hz
These frequency bands correspond to different functional states of the brain.

Awake: Dominated by alpha and beta waves, indicating quick, small changes and less synchrony. Neurons are processing information independently.
Falling Asleep: Slower waves (theta) appear, indicating more synchrony.
Stage One Sleep: Dominated by theta waves. Transitional stage. Eyes may still open, and individuals may not realize they were sleeping. Lasts about 10 minutes.
Stage Two Sleep: Features K-complexes (large, irregular deviations) and sleep spindles (short, definable features). A transition phase usually of 10 to 15 minutes. Sleep spindles persist in later stages.
Stage Three and Four Sleep (Slow Wave Sleep): Dominated by delta waves, indicating large, slow waves with high synchrony. Considered deep sleep. Brain is in a state incompatible with processing external information, focused on internal housekeeping and recovery.
REM Sleep: EEG looks similar to wakefulness (paradoxical sleep). Rapid eye movements occur, and skeletal muscles are paralyzed.
There are transition stages, but the three main states to consider are wakefulness, slow wave sleep, and REM sleep.

Brain is very active, resembling wakefulness in EEG patterns.
Eyes move rapidly under closed eyelids (Rapid Eye Movement).
Skeletal muscles are completely relaxed and paralyzed, a phenomenon called atonia.
Dreams are most likely to occur during REM sleep.

Slow Wave Sleep: Brain is in a synchronized state, recovering from daytime activity. Muscles can move, but you're less aware of surroundings.
REM Sleep: Brain is active. Muscles are paralyzed, and dreams occur.

A typical night's sleep involves cycling between REM and slow wave sleep, with each cycle lasting about 90 minutes.
As the night progresses, the proportion of REM sleep increases, while the proportion of slow wave sleep decreases.
Waking up during REM sleep results in remembering a dream. Waking up from slow wave sleep happens in a confused state.

The separation between electrodes and the brain limits EEG's spatial resolution.
EEG is reliable for distinguishing sleep phases because the entire brain typically exhibits similar activity during these phases.
Different wave frequencies correlate with different brain states and sleep stages.

The following lectures will focus on the neural mechanisms that regulate transitions between wakefulness and different sleep states.

Multiple brain areas are responsible for maintaining alertness and wakefulness.
Many of these areas are located in the brainstem reticular formation.
The five neurotransmitters that are involved in keeping us alert and awake are: acetylcholine, noradrenaline, serotonin, histamine, and hypocretin.
The basal forebrain and hypothalamus are critical regions as well.

Two groups of cholinergic neurons are important:
- One in the mesencephalon (pons), part of the reticular activating system.
- One in the basal forebrain.
These neurons have long axons that release acetylcholine throughout the brain, influencing most cortical neurons.
Acetylcholine release is high during wakefulness and REM sleep but low during slow wave sleep.

The locus coeruleus (in the reticular activating system in the pons) uses noradrenaline.
The locus coeruleus is particularly active during vigilance triggered by external stimuli.
Like cholinergic neurons, the locus coeruleus has long axons that influence a large part of the brain.
Activity in the locus coeruleus drops during sleep and is lowest during REM sleep.

Serotonin originates from the raphe nuclei, also part of the reticular activating system.
The raphe nuclei release serotonin throughout the brain.
Serotonin is involved in internally driven vigilance and alertness.
Serotonin levels are low during sleep, especially REM sleep.

Histamine is used by neurons in the tuberomammillary nucleus.
Histamine is high during wakefulness and low during sleep.
Antihistamines can cause drowsiness if they cross the blood-brain barrier, an important consideration for people taking allergy medicine.

Hypocretin is released by neurons in the lateral hypothalamus.
Hypocretin increases activity in the locus coeruleus, raphe nuclei, tuberomammillary nucleus and basal forebrain, the dorsal pons, and the cortex.
Hypocretin serves as a master controller, activating other areas that promote wakefulness.
Hypocretin levels drop during slow wave sleep and REM sleep.