Wakefulness, sleep, the EEG and seizures !

Regulation
Exam priority: medium

The brain cycles between waking, deep sleep and dream sleep; the EEG shows each state as a different rhythm.

Updated 2026-11-01

Also called: Sleep stages, REM sleep, Electroencephalogram

See it in 3D: Thalamus · Pons · Midbrain · Hypothalamus · Pineal gland · White matter

What it is

Wakefulness depends on arousal systems in the brainstem and hypothalamus that keep the thalamus and cortex active. Sleep comes from inhibition of those systems, pushed by accumulated adenosine (sleep pressure) and timed by the suprachiasmatic nucleus (circadian clock). The EEG records the summed synaptic potentials of cortical pyramidal cells; a seizure is an abnormal, excessive and synchronous discharge of neurons.

Action potential

Rest −70 mVThreshold −55 mVPeak +35 mVDuration ≈ 1–2 ms

Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.

Mainly Na⁺
Mainly K⁺
Rest
Threshold
ARP: absolute refractory period
RRP: relative refractory period

The EEG sums synaptic potentials, but seizures start when many neurons fire action potentials together. Tap a phase.

Regulation

+Ascending arousal system (noradrenaline from locus coeruleus, serotonin from raphe, acetylcholine from pons and basal forebrain, histamine): Activates thalamus and cortex: desynchronized waking EEG.
+Orexin (hypocretin) from the lateral hypothalamus: Stabilizes wakefulness; its loss causes narcolepsy.
−Ventrolateral preoptic nucleus (GABA): Inhibits arousal centres and starts sleep.
−Adenosine accumulated during waking: Raises sleep pressure; caffeine blocks its receptors.
−Melatonin (darkness signal via the suprachiasmatic nucleus and pineal): Signals night and helps time sleep onset.

Sleep stages in a healthy adult

Stage	EEG	Share of night	Key features
Awake, eyes closed	Alpha	—	Relaxed wakefulness
N1	Theta	≈5%	Drowsy transition, easy to wake
N2	Spindles and K-complexes	≈45–55%	Most of total sleep
N3	Delta (slow waves)	≈15–20%	Deepest; growth hormone; sleepwalking, night terrors, bedwetting
REM	Fast, low voltage (like waking)	≈20–25%	Dreams, eye movements, muscle atonia, erections

Clinical notes

Obstructive sleep apnea fragments sleep and lowers oxygen saturation repeatedly: daytime sleepiness, hypertension, arrhythmias.
Narcolepsy: loss of orexin neurons; REM intrudes into waking (cataplexy, sleep paralysis, hallucinations).
Status epilepticus is an emergency; prolonged seizures damage neurons through excitotoxicity.
After traumatic brain injury, seizures may appear early or months later.

Test yourself

Think of the answer, then tap the question to see it.

What does the EEG actually record?

The summed excitatory and inhibitory postsynaptic potentials of large populations of cortical pyramidal cells aligned in parallel, not individual action potentials.

Why does REM sleep look like waking on the EEG but the person does not act out dreams?

Cortex is active (desynchronized EEG), but pontine circuits inhibit spinal motor neurons, producing atonia; its failure causes REM sleep behaviour disorder.

Why does caffeine reduce sleepiness?

It blocks adenosine receptors. Adenosine builds up during waking and inhibits arousal systems, so blocking it removes part of the sleep pressure.

A child has brief staring spells many times a day; the EEG shows 3 Hz spike-and-wave. Which mechanism is involved?

Absence seizures: abnormal rhythmic oscillation of thalamocortical circuits, driven by T-type calcium channels in thalamic neurons.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

In Biochemistry

Glutamate (glutamic acid)

← All of physiology

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 13.2 The Central Nervous System (OpenStax (Rice University), 2022); Obstructive Sleep Apnea (StatPearls (NCBI Bookshelf), 2024); Generalized Tonic-Clonic Seizure (StatPearls (NCBI Bookshelf), 2024); Physiology, Pineal Gland (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 60). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.