Sleep neuroscience
Stages and their circuits, the two-process model, replay and memory consolidation, and what closed-loop stimulation is trying to do. The biology behind the hypnogram your band drew.
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Your band recorded a night and the stager drew a staircase: wake, light sleep, deep sleep, back up, REM, repeat every ninety minutes, with the deep sleep front-loaded and the REM back-loaded. Each step of that staircase is a different brain state with a different circuit in charge, and the sequence is not arbitrary. Sleep is when the brain does maintenance it cannot do while awake, and the rhythms you detected are the maintenance in progress.
The stages, mechanistically
Wake. Brainstem and hypothalamic arousal systems (noradrenaline, acetylcholine, histamine, orexin) bathe the cortex and thalamus in neuromodulators that keep neurons depolarized and desynchronized. Low-amplitude, fast EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry; alpha with eyes closed.
N1. Arousal drive drops. Alpha fades; slow rolling eye movements; theta. Two minutes, easily interrupted, and the stage every stager gets wrong because it is a transition.
N2. The thalamus, no longer held awake, begins to gate: spindlesSleep spindle A half-second burst of 11 to 16 Hz activity in light sleep, generated in the thalamus. Related: HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG., Thalamocortical loopThalamocortical loop The two-way circuit between the thalamus and the cortex whose rhythmic activity produces alpha, sleep spindles, and other EEG rhythms.. Glossary entry from the reticular-relay loop, and K-complexes, single large slow wavesSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry. Half the night is N2.
N3. Cortex falls into up and down states: slow wavesSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep., HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG.. Glossary entry, delta, the largest EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry there is. Hard to wake from. Concentrated in the first two cycles because the pressure to enter it, built up during the day, is highest then.
REM. A cholinergic brainstem population switches on, the cortex becomes as active as in wake, the eyes dart, dreams are vivid, and a separate circuit paralyzes the skeletal muscles so the dreams are not acted out. EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry looks like wake with no alpha; the chin EMGElectromyography (EMG) Recording the electrical activity of muscles, hundreds of microvolts to millivolts, much larger than EEG. Related: Myoelectric prosthesisMyoelectric prosthesis An artificial limb controlled by EMG from the remaining muscles. , Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. . Glossary entry goes flat; that is how the stager separates the two.
Why the staircase has that shape
The two-process model: a homeostatic pressure S that rises during wake and falls during sleep, and a circadian rhythm C from the hypothalamus’s clock that gates when sleep is permitted. At bedtime S is high, so deep sleep comes first and hard. As S discharges, N3 shrinks and REM, which C favours toward morning, grows. Caffeine blocks the adenosine that carries S; jet lag is C out of phase with the schedule; both show up in the hypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. Related: Polysomnography (PSG)Polysomnography (PSG) The overnight sleep study: EEG, EOG, chin EMG, breathing, oxygen, and heart rate recorded together and scored in 30-second epochs. , Sleep-EDFSleep-EDF A public PhysioNet dataset of whole-night sleep recordings with expert stage labels, the standard training set for automatic sleep staging. . Glossary entry.
During deep sleep the hippocampus replays, at high speed, the sequences of activity from the day. Where in the slow wave does this replay tend to occur?
In the up state, nested with the spindle. Hippocampal sharp-wave ripples carrying replay occur preferentially during the cortical up state and in the troughs of spindles, so that the reactivated memory arrives at cortex when its synapsesSynapse The junction where one neuron signals to another, usually by releasing a chemical that opens channels on the next cell. Related: Synaptic plasticitySynaptic plasticity The strengthening or weakening of synapses with use, the physical basis of learning. , NeurotransmitterNeurotransmitter A chemical released at a synapse that changes the next neuron's membrane potential. . Glossary entry are ready to change. That three-way nesting (slow waveSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry, spindle, ripple) is the leading account of how memories move from hippocampus to cortex overnight, and it is the reason enlarging slow wavesSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry with sound is expected to help memory.
Consolidation
Learn a list of words before sleep and you recallRecall A correction or removal of a marketed device; FDA's public database of them is a catalogue of how designs fail people. Glossary entry more of them in the morning than after an equal time awake; the effect tracks the amount of slow-wave sleep. Rodent recordings show hippocampal place-cell sequences from the day replayed during subsequent sleep, compressed twenty-fold. The active systems consolidation account says the hippocampus, a fast-learning temporary store, teaches the cortex, a slow permanent one, by replaying during slow wavesSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry. A complementary account, synaptic homeostasis, says slow-wave sleep renormalizes synapsesSynapse The junction where one neuron signals to another, usually by releasing a chemical that opens channels on the next cell. Related: Synaptic plasticitySynaptic plasticity The strengthening or weakening of synapses with use, the physical basis of learning. , NeurotransmitterNeurotransmitter A chemical released at a synapse that changes the next neuron's membrane potential. . Glossary entry that were potentiated during the day, pruning noise and preserving signal. Both are probably partly right, and the closed-loop protocol you built is a test of the first.
What closed-loop stimulation does
Two soft clicks timed to the up-state of a detected slow waveSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry, and the following slow wavesSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry are larger and the spindles more numerous, and next-morning recallRecall A correction or removal of a marketed device; FDA's public database of them is a catalogue of how designs fail people. Glossary entry improves in most (not all) studies. The stimulation is doing what the brain does anyway, a little more strongly, at the moment the brain is already doing it. Clicks out of phase have no effect or a negative one. This is the model for a whole class of neurotechnology: sense a brain state, act only in phase with it, and let the brain’s own mechanisms do the work.
Clinical sleep
Obstructive sleep apnea, where the airway collapses and the sleeper wakes repeatedly, is the most common diagnosis in a sleep lab and the reason polysomnography records breathing and oxygen. Insomnia, narcolepsy (orexin loss), REM behaviour disorder (the paralysis fails; it predicts Parkinson’s disease years ahead), and restless legs round out the clinic. A sleep technologistSleep technologist The person who runs overnight sleep studies and scores them; a certification path an undergraduate can complete and work nights. Related: Polysomnography (PSG)Polysomnography (PSG) The overnight sleep study: EEG, EOG, chin EMG, breathing, oxygen, and heart rate recorded together and scored in 30-second epochs. . Glossary entry sees all of them, which is why the clinic thread recommends the job.
Deep dive Why we sleep, honestly 2 min
Nobody knows, fully. Consolidation, synaptic renormalization, metabolic clearance (the glymphatic system flushes the brain’s interstitial space during slow-wave sleep), energy conservation, and immune function all have evidence. Every animal with a nervous system does something like it, and total deprivation kills rats in weeks. That a function so universal is so unexplained is a good reminder of how early the field is.
Deep dive Consumer sleep tech and what it can measure 2 min
Wrist devices infer stages from movement and heart rate, which correlate with sleep stage but do not define it; their agreement with polysomnography is modest, especially for N1 and for distinguishing REM from wake. Forehead EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry bands measure the actual defining signal and do much better. The regulatory line between wellness product and medical device runs right through this space, which Phase 5 covers.
Explain what this page was about to your roommate in three sentences. No jargon they would not know.
We move through light sleep, deep sleep with large slow waves, and dream sleep about every 90 minutes, with deep sleep early in the night and dream sleep late. We consolidate memory when the deep sleep up state holds spindles of 11 to 16 hertz together with fast hippocampal replay compressed about twentyfold, sending the day back to cortex. We enlarge those slow waves with two soft clicks timed to the up state, and morning recall improves in most studies while clicks out of phase do not help.
Yours does not have to match. Check that the three ideas are there.