HUMAN · SLEEP

ONE NIGHT

Eight hours, minute by minute. Scrub the night and watch the brain change gear — and see what each stage is for.

23:42N3 · DEEP SLEEPcycle 1 of 5

EEGEYESCHIN50 µV · 1 s grid

WAVES: slow waves 0.5–2 Hz
SIZE: over 75 µV peak to peak, at least 20% of the time
OF THE NIGHT: about 13–23%, mostly in the first half of the night · this night 20%

Huge slow waves roll across the cortex, usually travelling front to back. Whole populations of neurons fall silent together, then fire together, about once a second. It is the hardest stage to wake from, and the one that shrinks most with age.

Memory. In N2 and deep sleep, slow waves, spindles and ripples in the hippocampus tend to arrive together, and the leading theory is that this is the day being replayed into long-term storage in the cortex — the cyan sparks. Strong evidence, still being worked out.

In sleeping mice, the space between brain cells widened by about 60% and waste was flushed out faster (Xie et al., 2013). Whether sleep speeds brain clearance the same way in people is still actively debated.

23:42