Foreword Barry Jacobs once memorably commented that the problem in understanding the role of serotonin in the brain was that it was implicated in virtually eve- thing but responsible for nothing 1]. Serotonin (5-HT) is defnitely implicated in the regulation of sleep but what does it do? Once seen as a sleep-promoting molecule, Jacob s own work, lucidly described in this volume, radically changed our view. In fact, serotonin neurons are most active during waking; their - tivity diminishes with sleep onset and disappears during rapid eye movement (REM) sleep. Serotonin, therefore, is...