Sleep Science: Stages, Circadian Rhythm, and Why We Sleep

Humans spend roughly one-third of their lives unconscious — and we still don't fully understand why. What is clear is that the consequences of disrupted sleep extend from impaired cognition to suppressed immune function to accelerated neurodegenerative disease. Sleep is not a passive pause in life but one of the most complex and active biological processes known.

1. Sleep Stages and Architecture

An EEG (electroencephalogram) measures brain electrical activity and reveals that sleep is not a uniform state but cycles through distinct stages roughly every 90 minutes:

Sleep stages (AASM 2007 classification): Wake: High-frequency (~10-30 Hz beta waves), low-amplitude, desynchronised NREM Stage 1 (N1): ~5% of sleep. Transition to sleep. Theta waves (4-8 Hz). Hypnic jerks may occur (sudden muscle contractions). NREM Stage 2 (N2): ~45-55% of sleep. Sleep spindles (12-15 Hz bursts, 0.5-3 s) generated by thalamocortical loops. K-complexes (large single waves). Consolidation of procedural memory. Temperature and heart rate drop. NREM Stage 3 (N3, "slow-wave sleep" / SWS): ~15-20% of sleep. Slow oscillations (0.5-1 Hz), delta waves (0.5-4 Hz). Highest arousal threshold — hardest to wake from. Proportional to prior wake time (homeostatic marker). Functions: memory consolidation, glymphatic clearance, GH release. Predominates early in the night. REM (Rapid Eye Movement): ~20-25% of sleep. Nearly-awake EEG, muscle atonia. Characteristic: rapid conjugate eye movements, vivid dreaming. Hippocampal theta oscillations (4-8 Hz). Proportional to prior REM deprivation (REM rebound). Functions: emotional memory processing, creative insight, motor learning. Predominates late in the night / morning hours. A typical 8h night: ~4-5 complete cycles. First half: N3 dominant; second half: REM dominant.

2. Circadian Rhythm and the SCN

The suprachiasmatic nucleus (SCN) — a paired structure of ~20,000 neurons in the hypothalamus — acts as the master circadian clock, with a free-running period of ~24.2 hours that must be entrained daily:

Molecular clock: A transcriptional-translational feedback loop. CLOCK and BMAL1 proteins drive transcription of PER (Period) and CRY (Cryptochrome) genes. PER/CRY proteins accumulate and inhibit CLOCK/BMAL1 activity. Full cycle: ~24 h. This same loop (with homologs) operates in virtually every cell of the body — the peripheral clocks.
Light entrainment: Intrinsically photosensitive retinal ganglion cells (ipRGCs) containing melanopsin (peak sensitivity ~480 nm — blue light) project to the SCN via the retinohypothalamic tract. Light at the wrong circadian phase shifts the clock. Morning light advances the clock; evening light delays it.
Melatonin: Secreted by the pineal gland under SCN control. Synthesis begins 2-3 hours before habitual sleep onset (DLMO — dim light melatonin onset). Suppressed by light (especially short-wavelength). Melatonin levels peak around 2-4 AM then decline. Exogenous melatonin (0.5-1 mg) can advance or delay circadian phase when timed correctly.

3. Sleep Pressure: Adenosine and Process S

Adenosine accumulation model: During wakefulness: neural activity \to ATP hydrolysis \to ADP \to AMP \to Adenosine Adenosine accumulates in the extracellular space, especially in the basal forebrain. Adenosine acts on A1 and A2A receptors \to inhibits wake-promoting neurons \to promotes sleep (somnogenic effect) Adenosine clears during sleep (via adenosine kinase and deaminase) \to "sleep debt" is biochemically real and measurable 40 hours without sleep \to adenosine reaches levels causing severe impairment. Cognitive performance after 21 days of 6h sleep/night = 2 nights total sleep deprivation. Caffeine mechanism: Caffeine = adenosine receptor ANTAGONIST (A1, A2A) Does NOT reduce adenosine accumulation — just blocks its effect. After caffeine clears (t½ \approx 5-6 h, varies by genome CYP1A2): Adenosine receptor block wears off \to accumulated adenosine floods receptors \to "caffeine crash" Important: caffeine does not prevent N3 slow-wave sleep loss — it only masks the subjective feeling of sleepiness. Neurological impairment continues even if you feel alert on caffeine with sleep deprivation.

4. The Two-Process Model

The Borbély-Achermann two-process model (1982) elegantly explains the timing of sleep and wakefulness through two independent processes:

Process S (homeostatic sleep pressure): S rises during waking (adenosine accumulation, SWA spectral power builds). S falls during sleep (especially N3). Modelled as exponential rise during wake, exponential decay during sleep. S determines the "depth" and drive for sleep — how much SWS occurs. Process C (circadian rhythm): Sinusoidal oscillation with period ~24 h, controlled by SCN. C determines propensity for wakefulness/sleepiness at particular times of day. Two "wake maintenance zones": Morning wake zone: ~08:00-10:00 (weak) Afternoon wake zone: ~18:00-20:00 (strongest — "forbidden zone for sleep") Post-lunch dip / siesta zone: ~14:00-16:00 (circadian trough) Sleep onset occurs when S crosses a threshold defined by C. Wake occurs when S has declined sufficiently AND C provides wake signal. Predictions confirmed: - Afternoon dip in alertness (circadian trough, not just post-lunch) - "Second wind" in evening despite accumulated S - Naps reduce subsequent SWS (reduce S), leave late-sleep REM largely intact - Shift work pathology: circadian misalignment forces sleep against wake signal

5. Why We Sleep: Functions of Sleep

Memory consolidation: SWS: hippocampal ripple-cortical slow oscillation dialogue transfers memories to neocortex (systems consolidation). REM: emotional de-contextualisation and integration of new information with existing schemas. Both stages necessary for optimal learning.
Glymphatic clearance: Iliff et al. (2013) discovered that during NREM sleep, interstitial space in the brain expands by ~60%, allowing CSF to flush along perivascular channels. This clears metabolic waste including amyloid-β (accumulates to form Alzheimer's plaques if not cleared). Sleep deprivation → ↑ amyloid accumulation — risk factor for Alzheimer's.
Immune function: Sleep is essential for immune memory. Sleep-deprived individuals show 50% lower antibody response to influenza vaccination (Spiegel 2002). Cytokine (IL-1β, TNFα) production increases during sleep and promotes NREM. Fever during illness increases SWS (adaptive).
Metabolic regulation: Sleep deprivation → increased ghrelin (hunger hormone), decreased leptin (satiety hormone) → appetite increase ~24%. HbA1c impaired, insulin resistance elevated. Short sleep duration strongly associated with obesity and Type 2 diabetes.
Cardiovascular: Daylight saving time transitions show 24% increase in heart attacks the Monday after losing 1 hour of sleep (Janszky 2012). Chronic short sleep (≤6h) doubles cardiovascular mortality risk.

6. Sleep Disorders

Insomnia disorder: Difficulty initiating or maintaining sleep, with daytime impairment, despite adequate opportunity. ~10-15% prevalence. CBT-I (Cognitive Behavioural Therapy for Insomnia) is first-line — more effective than pharmacology long-term. Core components: sleep restriction (temporarily limit time in bed to build sleep pressure), stimulus control, relaxation techniques.
Obstructive sleep apnoea (OSA): Upper airway collapses repeatedly during sleep → apnoea events (10-120 s), oxygen desaturation, arousal. Severity: AHI (apnoea-hypopnoea index) — events per hour. Mild <15, moderate 15–30, severe >30. CPAP (continuous positive airway pressure) is gold standard. Associated with hypertension, arrhythmias, cognitive impairment, EDS.
Narcolepsy type 1: Hypocretin (orexin) neuron loss (autoimmune — ~70,000 hypocretin neurons in lateral hypothalamus destroyed, probably by T-cells triggered by viral/environmental exposure). Symptoms: EDS, cataplexy (muscle atonia triggered by emotion), hypnagogic hallucinations, sleep paralysis. Direct REM sleep onset from wakefulness (REM without NREM — "sleep-onset REM periods").
REM sleep behaviour disorder (RBD): Loss of REM atonia → physical acting out of dreams (punching, kicking). 80-90% convert to Parkinson's disease or other synucleinopathy within 10-15 years. Considered a prodromal biomarker of neurodegeneration.

7. Chronotypes and Social Jetlag

Chronotype is the individual expression of circadian phase — the natural timing preference for sleep and wakefulness. It is ~50% heritable (PER3, CLOCK gene variants) and varies systematically with age:

Children: morning types (early phase) — typically earlier wake and sleep times.
Teenagers: dramatic late shift, peaking at 19-21 years old. Average DLMO in 17-year-olds is 2+ hours later than adults. A class starting at 8 AM is asking teenagers to perform at peak cognition at their equivalent of 6 AM.
Adults: continuous advance (earlier phase) with age. Elderly often have advanced phase syndrome.

Social jetlag: Roenneberg et al. (2006) — the misalignment between biological clock and social clock (work/school schedules). Most people sleep differently on workdays vs free days. The difference in mid-sleep time between workdays and free days is "social jetlag" — median ~1 hour in adults, up to 3+ hours in teenagers. Each hour of social jetlag is associated with 33% increased odds of obesity, increased risk of type 2 diabetes, depression, and all-cause mortality. School start time reform is the highest-impact, lowest-cost public health intervention with the strongest evidence base that schools consistently ignore.