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Explain me sleep physiology with diagram for my university exam

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Sleep Physiology - Complete University Exam Guide

Sources: Harrison's Principles of Internal Medicine 22E (2025), Adams and Victor's Principles of Neurology 12E, Bradley and Daroff's Neurology in Clinical Practice

1. Introduction & Measurement

Sleep is a reversible, periodically recurring state of reduced consciousness, responsiveness, and motor activity. Most adults require 7-9 hours per night. Sleep is studied using polysomnography (PSG), which continuously records three parameters:
ParameterWhat it measures
EEG (electroencephalogram)Brain electrical activity / cortical rhythm
EOG (electrooculogram)Eye movement activity
EMG (electromyogram)Chin + limb muscle tone

2. Sleep Stages (AASM 2007 Classification)

PSG defines two fundamental sleep states: NREM and REM sleep.

Stage W - Wakefulness

  • EEG: Alpha waves (8-13 Hz) when eyes closed; desynchronized low-voltage when eyes open
  • EMG: High muscle tone
  • EOG: Normal waking eye movements

NREM Sleep (75-80% of total sleep time)

NREM is subdivided into three stages based on EEG:

Stage N1 (3-8% of total sleep)

  • Transition from wake to sleep
  • EEG: Alpha waves diminish (<50% of epoch); replaced by theta waves (4-7 Hz) and beta waves
  • EOG: Slow, rolling eye movements
  • EMG: Slightly reduced
  • Vertex sharp waves appear toward end of N1

Stage N2 (50-60% of total sleep)

  • "Light sleep" - lasts ~30-60 min per cycle
  • EEG hallmarks: Sleep spindles (12-18 Hz, typically 14 Hz) and K complexes (biphasic high-amplitude sharp slow-wave complexes)
  • EOG: Minimal eye movements
  • EMG: Normally active

Stage N3 - Slow Wave Sleep / Deep Sleep (15-25% in young adults)

  • "Deep sleep" - highest arousal threshold
  • EEG: Delta waves (0.5-2 Hz, >75 µV) occupy >20% of epoch
  • Predominates in the first third of the night
  • Growth hormone secretion peaks during this stage
  • Most important for physical restoration
Polysomnographic recordings showing EEG patterns across wake and NREM stages:
Polysomnographic recordings from awake state through NREM sleep stages showing alpha rhythms in wake, slow rolling eye movements in N1, K-complexes and sleep spindles in N2, and delta waves in N3
Panel A = Awake (alpha waves, active EMG); Panel B = N1 (slow rolling eye movements, reduced alpha); Panel C = N2 (K-complex + sleep spindle clearly labeled)

REM Sleep (20-25% of total sleep)

Also called paradoxical sleep or fast-wave sleep:
  • EEG: Low-amplitude, mixed-frequency, desynchronized - paradoxically similar to wakefulness
  • EOG: Rapid eye movements in bursts/flurries
  • EMG: Atonia - nearly complete muscle paralysis (except respiratory muscles and extraocular muscles)
  • Dreams occur predominantly during REM
  • Autonomic instability: variable heart rate, blood pressure, and breathing pattern
  • REM cycles lengthen as the night progresses - last REM episode can last ~1 hour
Key paradox of REM: The brain is highly active (EEG resembles wakefulness) but the body is paralyzed.

3. Sleep Architecture (The Hypnogram)

A hypnogram is a graphical representation of sleep stages over the night. Key features:
  • After sleep onset, progression is N1 → N2 → N3 within 45-60 minutes
  • First REM episode occurs ~90-100 minutes after sleep onset
  • NREM-REM cycles repeat every ~90-100 min (4-6 cycles per night)
  • Early night: dominated by slow-wave sleep (N3)
  • Late night: dominated by longer REM episodes
  • After sleep deprivation, NREM recovers first, then REM
Sleep hypnogram comparing architecture across age groups:
Sleep architecture hypnograms comparing children, young adults, and elderly showing REM cycles (green), N1-N4 stages over 7 hours - deep sleep dominates early cycles in children and young adults, while elderly show fragmentation and loss of N3/N4
Note: Children have abundant deep sleep (stages 3-4). Young adults show classic architecture. Elderly show fragmented sleep with loss of N3 and more awakenings.
Age-related changes in sleep:
ParameterInfantsYoung AdultsElderly
Total sleep16-20 h7-8 h~6.5 h
REM %~50%20-25%Reduced
N3 sleepProminent15-25%Often absent
Sleep fragmentationPolyphasicMinimalIncreased

4. Neural Control of Sleep and Wakefulness

The Flip-Flop Switch Model

Two opposing systems govern sleep-wake transitions:

Ascending Arousal System (promotes WAKEFULNESS)

Clusters of neurons extending from the upper pons to hypothalamus and basal forebrain that activate the cortex, thalamus, and forebrain using:
  • Norepinephrine (locus coeruleus)
  • Serotonin (raphe nuclei)
  • Histamine (tuberomammillary nucleus)
  • Acetylcholine (pedunculopontine and laterodorsal tegmental nuclei)
  • Dopamine (ventral periaqueductal grey)
  • Glutamate
  • GABA (basal forebrain - inhibits cortical inhibitory interneurons, paradoxically promoting arousal)

Orexin (Hypocretin) System

  • Neurons in the lateral hypothalamus
  • Use the peptide orexin (hypocretin) to reinforce and stabilize arousal by activating other arousal-promoting cell groups
  • Loss of orexin neurons causes narcolepsy

Sleep-Promoting System

  • GABAergic neurons in the ventrolateral preoptic nucleus (VLPO) and pons
  • GABA inhibits all components of the arousal system, allowing sleep to occur
  • Additional melanin-concentrating hormone (MCH) neurons in lateral hypothalamus promote REM sleep
These two systems engage in mutual inhibition - like an electrical "flip-flop switch" - producing rapid, stable transitions between wake and sleep, avoiding prolonged intermediate states.
Brain circuit diagram - arousal system vs. sleep-promoting system:
Sagittal brain diagram showing the ascending arousal system (green neurons from brainstem to cortex/thalamus), orexin/hypocretin neurons in hypothalamus (blue), and GABAergic sleep-promoting system (red, preoptic area), with drug targets labeled
Green = Ascending arousal system (monoaminergic + cholinergic) | Blue = Orexin system (stabilizes arousal) | Red = GABAergic sleep-promoting system (VLPO)

REM Sleep Switch

A separate brainstem flip-flop switch governs NREM-REM transitions:
  • REM-Off neurons (lower midbrain, GABAergic) inhibit REM
  • REM-On neurons (upper pons): GABAergic (inhibit REM-Off) + glutamatergic (produce REM phenomena)
  • REM-On neurons projecting to medulla/spinal cord activate GABA/glycine interneurons → hyperpolarize motor neurons → REM atonia
  • REM-On neurons projecting to forebrain → dreams
  • Cholinergic input favors REM; monoaminergic (NE + 5-HT) input prevents REM
  • Damage to REM atonia neurons → REM Sleep Behavior Disorder

5. Regulation of Sleep - Two-Process Model

Sleep-wake timing is governed by three interacting processes:

Process S - Homeostatic Sleep Drive

  • Sleep pressure builds with prolonged wakefulness
  • Mediator: Adenosine accumulates during wakefulness
    • Acts on A1 receptors to directly inhibit arousal regions
    • Acts on A2a receptors to activate VLPO (sleep neurons)
  • Caffeine works by blocking A2a receptors (blocking sleepiness)
  • Other mediators: Prostaglandin D2
  • After sleep deprivation, N3 recovers first ("slow-wave sleep rebound")

Process C - Circadian Drive

  • Controlled by the suprachiasmatic nuclei (SCN) of the hypothalamus
  • Intrinsic period: ~25 hours, entrained to 24-h cycle by light (via retinohypothalamic tract)
  • SCN lesions cause complete disorganization of sleep-wake cycles
  • SCN regulates melatonin secretion by the pineal gland (darkness → melatonin rise → promotes sleep onset)

Process A - Allostatic (Stress) Inputs

  • Psychological stress, anxiety, and psychiatric disorders activate the ascending arousal system
  • Chronic insomnia involves hyperactivation of arousal pathways
  • Common cause of insomnia in anxiety disorders

6. Neurotransmitter Summary Table

NeurotransmitterDuring WakeDuring NREMDuring REM
Acetylcholine↑↓↑ (high)
Norepinephrine↑↓↓↓ (silent)
Serotonin↑↓↓↓ (silent)
Histamine↑↓↓↓
Orexin/Hypocretin↑↓↓↓
Dopamine↑~~ (minimal change)
GABA (VLPO)↓↑↑
AdenosineAccumulates↓↓
Source: Bradley and Daroff's Neurology in Clinical Practice

7. Physiological Changes During Sleep

SystemNREMREM
Muscle toneMildly reducedAtonia (paralysis)
Heart rateRegular, slowedVariable, irregular
Blood pressureReducedVariable
BreathingRegularIrregular
Temperature regulationMaintainedImpaired (poikilothermy)
Growth hormonePeak secretion (N3)Minimal
Penile/clitoral tumescenceAbsentPresent
DreamsRare/vagueVivid, narrative

8. Functions of Sleep (Important for Exams)

  • Memory consolidation: REM sleep consolidates procedural and emotional memories; N3 consolidates declarative memory
  • Metabolic restoration: N3 is associated with growth hormone secretion and anabolic processes
  • Immune function: Sleep deprivation impairs immune responses
  • Cognitive function: Sleep deprivation causes reaction time slowing and vigilance deficits
  • Metabolic regulation: Chronic sleep deficiency → glucose intolerance, obesity risk, metabolic syndrome
  • Cardiovascular health: Sleep deficiency linked to accelerated atherosclerosis and cardiac disease
  • CNS clearance: Glymphatic system flushes metabolic waste (including amyloid-beta) during sleep

Quick Exam Summary

NREM stages:
  N1 → Theta waves, slow rolling eye movements, vertex waves
  N2 → Sleep spindles + K complexes
  N3 → Delta waves (slow-wave sleep, deep sleep)

REM:
  Low-voltage mixed EEG, rapid eye movements, atonia
  Cholinergic; monoamines SILENT

Sleep cycle:
  N1 → N2 → N3 → N2 → REM (90-100 min/cycle, x4-6 per night)
  First third = N3 dominant
  Last third = REM dominant

Key structures:
  SCN → circadian pacemaker
  VLPO → GABAergic, promotes sleep
  Lateral hypothalamus → orexin (wake) + MCH (REM)
  Locus coeruleus → NE (wake-promoting)
  Adenosine → sleep pressure; caffeine blocks it
  • Harrison's Principles of Internal Medicine 22E (2025), pp. 253-254
  • Adams and Victor's Principles of Neurology 12E, pp. 411-414
  • Bradley and Daroff's Neurology in Clinical Practice, pp. 2305-2308
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