Sleep should not be considered a behavioural state characterized by brain inertia; instead, it is a highly dynamic process involving numerous brainstem areas and all physiological systems of the body. Our understanding of the underlying mechanisms responsible for sleep regulation has considerably advanced since the discovery of rapid eye movement (REM) sleep, about half a century ago. Based on standardized electroencephalographic, electro-oculographic and electromyographic features, two distinct main states periodically alternating throughout the night have been identified: REM and non-REM sleep; the latter is further distinguished into stages 1, 2, 3 and 4. Computerized analysis of sleep recordings yielded more detailed information on sleep physiology and pathology. Although still preliminary, neuroimaging studies promise to elucidate the functional alterations of neuronal substrates during sleep. Regarding sleep disorders, which account for a substantial individual and socio-economic burden, considerable progress has been achieved in terms of their classification, assessment, clinical diagnosis and treatment. Specific sleep disorders within the three major categories, that is, 'dysomnias', 'parasomnias', and 'sleep disorders associated with mental, neurologic, or other medical conditions', exhibit characteristic clinical features; sleep laboratory recordings considerably assist to definitely diagnose several among them. Pertinence of sleep medicine for psychiatrists is obvious, taking into consideration that psychiatric disorders account for the largest diagnostic group of patients with sleep problems. In fact, the basics of this interdisciplinary field should be of special concern both to medical students and clinicians of diverse backgrounds who are interested in acquiring the necessary skills to globally and comprehensively understand and eventually effectively treat their patients.