Sleep Medicine Reviews, Vol. 00, No. 0, pp 1±19, 2002 doi:10.1053/smrv.2001.0202, available online at http://www.idealibrary.com on Awakening from sleep T. A Ê kerstedt 1 , M. Billiard 2 , M. Bonnet 3 , G. Ficca 4 , L. Garma 5 , M. Mariotti 6 , P. Salzarulo 7 and H. Schulz 8 1 Karolinska Institute, Stockholm, Sweden, 2 Gui de Chauliac Hospital, Montpellier, France, 3 Dayton Department of Veterans Affairs, Medical Center, USA, 4 University of Naples, Italy, 5 H^ opital de la Salpe Ãtrie Áre Paris France, 6 University of Milano, Italy, 7 University of Florence, Italy, 8 Helios Klinikum, Erfurt, Germany Summary Awakening is a crucial event for the organism. The transition from sleep to waking implies physiological processes which lead to a new behavioural state. Spontaneous awakenings have varying features which may change as a function of several factors. The latter include intrasleep architecture, circadian phase, time awake, age, or disordered sleep. Despite its clear theoretical and clinical importance, the topic of awakening (in humans) has received little attention so far. This contribution focuses on major issues which relate to awakening from both basic (experimental) and clinical research. Recent knowledge on neurophysiological mechanisms is reported. The experimental data which provide in the human suggestions on the regulation of awakening are discussed, mainly those concerning sleep architecture and homeostatic/circadian factors also in a life-span perspective, since age is a powerful factor which may in¯uence awakening. Clinical contributions will examine two main sleep disorders: insomnia and hypersomnia. Daytime functioning is shown in insomniac patients and compared to other pathologies like sleep apnea. A ®nal section evokes links between some types of night waking and psychological factors. & 2002, Elsevier Science Ltd. All rights reserved. INTRODUCTION Awakening is a crucial event for the organism, sepa- rating sleep and wakefulness. The transition from sleep to waking implies physiological processes which lead to a new behavioural state. Transitions between sleep and wakefulness in humans encompass a wide spectrum of state changes, different in duration and characteristics: (i) ultrashort arousals in the range of seconds, (ii) short spontaneous awakenings from sleep in the range of minutes, and (iii) longlasting and consistent awake- nings, often representing the termination of a full sleep episode. In fact both terms, awakening and arousal, are used in the literature often with similar, even identical meaning. An epistemological discussion about the right to use one of these terms is beyond the scope of this review. However it could be remarked that ``arousal'' is mainly used in neurophysiology to indicate cortical events and in clinical domain for physiological events linked to respiratory pathology; awakening includes a behavioural component. Non-provoked awakenings, either ®nal or intra- night, have varying features: their number, temporal KEYWORDS awakening, thalamus, sleep architecture, time of day, biological rhythms, aging, development, insomnia, hypersomnia, dreams 1087±0792/02/000000  00 $35.00/0 & 2002, Elsevier Science Ltd. All rights reserved. Correspondence should be addressed to: Prof. Piero Salzarulo, Dept. of Psychology, University of Florence, Via S. Niccolo Á 93, 50125 Florence, Italy. E-mail: salzarulo@psico.uni®.it