Why we sleep

Sleep is an essential physiological process. Our bodies may be quiet and largely immobile, but our brains are highly active. Metabolites and other toxins that have accumulated over the course of the day get cleared away. Recent memories become consolidated. Scientists have thus begun to investigate the restorative and beneficial functions of sleep in great detail. See the special section beginning on page 550 .

Eukaryotic plankton biogeography and its determinants at global scale reflect differences in ecology and body size.

Exceptional increases in fluvial sediment fluxes in a warmer and wetter High Mountain Asia The flux of sediments from rivers in High Mountain Asia has increased substantially over the past 60 years.

Jenny L. Donelan, Catriona A. Morrison, Iain R. Barr & Jennifer A. Gill