Not sleeping is a form of torture. Without sleep, we die. We divert roads and close airports to protect it. But, for all its importance, do we know what sleep actually is? When we sleep, our intellect is paused. Sleep negates the very organ we use for synthesising experience as knowledge. It cannot be described while it’s happening: there can be no commentary as there is with, say, pain or emotion. Neither the conscious nor the unconscious mind can ever honestly observe: ‘I am asleep now.’ Here’s the conundrum: asleep, we cannot communicate the experience. Awake, we cannot experience it. The texture of sleep remains unknown.

In neuroscientific disciplines—that area of science that deals with the brain—sleep can be understood as a specific electrical state. The brain is an electrical organ, made up of neurons, each a tiny vector on a complex electrical network. All sleeping brains, be they infant or elderly, human or animal, exhibit notably similar electrical characteristics. Electroencephalography (EEG) uses sensitive receptors placed on the scalp to measure the summation of these billions of electrical signals and charts them side-by-side on reams of graph paper: a symphonic score of waveforms. From these lines, patterns emerge.¹ Delta waves, theta waves, K-complexes and intricate sleep spindles are the key landmarks of sleep’s geography, identifiable among the complex background patterns. The presence and frequency of these waveforms are signatures, defining the four stages of sleep. The stages are like steps at a swimming pool: we enter at wakefulness and wade down into the depths.