Pioneering 19th and early 20th-century research that proved our bodies follow biological rhythms and revealed the true cost of mental fatigue.
Chapter 1From ancient speculation to Gierse's 1842 temperature measurements - how the study of human performance transformed from folklore to physics.
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Chapter 2How experiments from 1890-1904 proved sleep deprivation kills faster than starvation and systematically destroys human cognition.
Read chapterThe search for a chemical cause of tiredness, and the theory that fatigue is a kind of self-poisoning that builds up while we are awake.
The discovery that attention comes and goes in cycles, and the first hard evidence that focus has built-in biological limits.
Audacious isolation experiments, deep underground and in constant daylight, that tried and failed to break the body's internal clock.
From the discovery of REM sleep to the realisation that a roughly 90-minute cycle keeps running through the day, not just at night.
How decades of scattered findings were pulled together into a single, authoritative theory of sleep and human rhythm.
How modern studies confirmed that the rhythm runs while we are awake, and settled on the term that defines it: the ultradian rhythm.