Introduction The human sleep-wake cycle is controlled by two central timekeeping mechanisms: a central circa- dian clock and a sleep homeostat. The primary circadian clock within the suprachiasmatic nucleus (SCN) generates a circadian signal that controls the circadian timing of behavioral and physiological rhythms, and also couples these processes to the outside world. The sleep homeostat maintains a day- to-day constancy in the level of sleep by determining current sleep needs (pressure) based on the history of prior wakefulness. One of the long established models of sleep regulation, the two-process model (Borbely, 1982; Daan et al., 1984; Achermann et al., 1993) proposes that slow wave activity (SWA) reflects the discharge of sleep pressure (hereafter referred to as ‘S’) that rises in proportion to prior wakefulness. During habitual sleep (i.e. 7-8 hours of sleep), sleep pressure (SWA, SWS) shows peak values shortly after sleep onset and then declines during successive nREM periods, reaching a lower asymptote after 4-5 sleep cycles (Feinberg, 1974; Dijk et al., 1990). Consistent with the function of the sleep homeostat, sleep deprivation (Dijk et al., 1993) and napping (Werth et al., 1996) increase and decrease, respectively, levels of S as reflected by the level SWA, and by the rate of appearance of SWA (RoS) at the beginning of sleep. In some circumstances however, such as during extended sleep and during forced desynchrony studies, levels of SWA rise when sleep pressure A biphasic daily pattern of Slow Wave Activity during a two-day 90-minute sleep wake schedule W.C. DUNCAN 1 , G. BARBATO 1,2 , I. FAGIOLI 3† , D. GARCIA-BORREGUERO 4 , T.A. WEHR 1 1 Mood and Anxiety Disorders Program, IRP, NIMH, Bethesda (MD), USA; 2 Department of Psychology, Second University of Naples, Italy; 3 Department of Psychology, University of Florence, Italy; 4 Department of Neurology, Fundacion Jimenez Diaz, Madrid, Spain ABSTRACT Twenty-four hour sleep patterns were measured in six healthy male volunteers during a 90-minute short sleep-wake (SW 30:60) cycle protocol for 48 hours. Sleep pressure estimates (amount of Slow Wave Sleep [SWS], SWA, and Rate of Synchronization [RoS: the rate of SWA build-up at the beginning of the NREM period]) were compared with the 24-hour patterns of body temperature (Tb 24 ) and sleep propensity. A moderate sleep debt was incurred over the 48 hour study as indicated by decreased levels of 24 hour sleep. On day 1, ultradian patterns of REM and SWS sleep were prominent; on day 2, more prominent were circadian patterns of REM sleep, SWS, Sleep Latency, TST and Tb 24 . Also on Day 2, biphasic patterns of SWA and RoS were expressed, with peaks occurring during the falling and rising limbs of Tb 24 . The biphasic peaks in SWA and RoS may be associated with phase-specific interactions of the circadian pacemaker with the sleep homeostat during conditions of moderate sleep pressure. Further research is needed to replicate the finding and to identify biological factors that may underlie the twelve hour pattern in SWA. Key words SWA • SWS • Rate of Synchronisation • Circadian rhythm • 30:60 sleep-wake schedule Corresponding Author: Wallace C. Duncan, Jr., Mood and Anxiety Disorders Program, NIMH, NIH, Bethesda, MD 20892, USA - Tel.: 301 496 9783 - Fax: 301 496 5439 - Email: wduncan@mail.nih.gov † Deceased 18 July, 2003. Archives Italiennes de Biologie, 147: 117-130, 2009.