Sleep and the brain.

Sleep is a fundamental behavior ubiquitous in the animal kingdom, necessary for the support of physical health and in humans for the maintenance of cognitive function. While it influences all body systems, it is particularly important for the brain and is typically characterized using measures of brain electrical activity. Sleep undergoes predictable changes across the lifespan, with notably dramatic alterations occurring during adolescence and with old age. Over and above the normal development changes, however, upwards of a third of the adult population experience some form of insomnia on a regular basis. This issue's special section on "Sleep through the Ages" contains papers addressing the neurological and neuropsychological implications of sleep in adolescents, older adults and insomnia sufferers, highlighting relations of sleep with brain structure and function.