The Biology of Night: Why We Sleep in Cycles and What Dreams Are For

In 1953, graduate student Eugene Aserinsky, working in Nathaniel Kleitman's laboratory in Chicago, noticed something unusual on the EEG of a sleeping child. The eyes were moving under closed lids — quickly, chaotically, as if tracking something. The brain waves at that moment resembled wakefulness rather than sleep. Aserinsky woke the subjects at this point — and they described dreams.

This is how REM sleep was discovered — Rapid Eye Movement. It turned out that our nightly sleep is not a uniform state, but a complex architecture of several alternating phases. Each performs its own function. And understanding this architecture changes how we think about sleep.

The Four Stages of Sleep: What Happens in Each

The current sleep classification (AASM, 2007) identifies four stages:

• N1 (transitional): the first 5–10 minutes after falling asleep. Muscles relax; hypnagogic jerks may occur (the sudden sensation of falling). Brain waves slow from beta/alpha to theta.
• N2 (light sleep): approximately 45–55% of total sleep time. Characteristic sleep spindles (12–15 Hz) and K-complexes appear — the brain actively suppresses responses to external stimuli and begins memory consolidation.
• N3 (deep, slow-wave sleep): the most valuable stage for physical recovery. Delta waves (0.5–2 Hz) dominate. Growth hormone release, tissue repair, glymphatic system activity — all of this happens here. The hardest stage to wake someone from.
• REM (rapid eye movement): the brain is nearly as active as during wakefulness, but the body is paralysed — a specialised mechanism (REM atonia) blocks motor neurons so we do not physically 'act out' our dreams. This is where primary consolidation of emotional memories occurs, and where creative 'recombination' of information takes place.

Why Cycles — and Why There Should Be Five

Over the course of a night, we pass through 4–6 cycles, each lasting approximately 90 minutes. But they are not identical. In the first half of the night, N3 dominates — deep sleep that restores the body. In the second half, cycles become richer in REM — the brain switches to emotional and cognitive processing.

This is why losing 1–2 hours of sleep disproportionately impairs cognitive function: you are losing primarily the REM-rich later cycles. An alarm at 6 am instead of 8 is not 'just two hours' — it is cutting off the two most REM-dense cycles of the night.

A historical note: The discovery of REM transformed medicine. Before Aserinsky and Kleitman, sleep was thought to be simply 'switching off' the brain. Afterwards, it became clear that the sleeping brain performs specialised work that cannot be replaced by waking rest. This discovery became the foundation of all modern sleep medicine and sleep neuroscience.

When Sleep Architecture Breaks Down: Apnoea and Genetics

Obstructive sleep apnoea (OSA) — pauses in breathing during sleep due to collapse of the soft tissues of the throat — affects approximately 10–15% of adults. It literally destroys sleep architecture: the person never reaches deep stages, and the night becomes a series of micro-arousals they do not remember.

Genetic predisposition to apnoea is substantial: the heritability of OSA is estimated at 40–60%. Key genetic factors include facial skeletal anatomy (genes determining jaw size and upper airway width), pharyngeal muscle tone, and central respiratory control.

— Continued in PRO Material —

The PRO material contains the full longreid 'Sleep Architecture': a detailed breakdown of each stage, what happens to the brain and body, how to read your smartwatch sleep data, and when it lies.

Premium contains the article 'Dreams and Evolution': why the brain needs dreaming from a biological standpoint, what happens in REM at the neural level, and why nightmares are not pathology but adaptation.

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