The core metaphor of my book, The Head Trip. The brain is a wheel, and consciousness is a pliant membrane pressed into the rim. Consciousness is always there – it never moves. But the wheel moves, spun by our biological clocks. The changes of state we move through at night are fairly proscribed; things are much looser during the day, though our general levels of arousal do rise and fall on a strong circadian tide. Magnified many times, the surface of the wheel is not smooth: it has a jagged circumference, a shifting continuum of sharp, raised bumps and crooked dents. This is brain activity; it pushes up from below, changing the contour of consciousness from one moment to the next.