Brain Research 866 (2000) 313–325 www.elsevier.com / locate / bres Interactive report Intracerebral hemodynamics probed by near infrared spectroscopy in 1 the transition between wakefulness and sleep a, b a a * Arthur J. Spielman , Gang Zhang , Chien-Ming Yang , Paul D’Ambrosio , a a a b Shiro Serizawa , Massanori Nagata , Hans von Gizycki , Robert R. Alfano a Sleep Disorders Center, Department of Psychology, The City College of the City University of New York, 138th Street and Convent Avenue, New York, NY 10031, USA b Institute for Ultrafast Spectroscopy and Lasers, New York State Center for Advanced Technology for Ultrafast Photonic Materials, Applications Department of Electrical Engineering and Physics, The City College of the City University of New York, 138th Street and Convent Avenue, New York, NY 10031, USA Accepted 29 March 2000 Abstract Previous imaging studies have shown that cerebral metabolism is gradually reduced at the beginning of sleep. Few studies have examined the sleep state transition periods from wakefulness to sleep and sleep to wakefulness. The current study used the Near Infrared Spectroscopy (NIRS) technique to describe the intracerebral hemodynamics at the frontal pole in the circumscribed period between wakefulness and sleep. Nine healthy young adults were studied during afternoon naps. Optical probes were placed on the forehead and EEG electrodes on the scalp. At sleep onset oxygenated hemoglobin (oxy-Hb) was reduced ( P,0.01) and deoxygenated hemoglobin (deoxy-Hb) showed a near significant reduction ( P,0.063). At sleep offset there were increases in oxy-Hb ( P,0.005) and deoxy-Hb ( P,0.05). In 18 of 26 transitions to sleep there was a coordinated fall in both NIRS parameters, we call the Switch Point, that lasted a mean of 3.6 s. In 32 of 36 transitions to wakefulness there was an analogous Switch Point that lasted a mean of 3.4 s. Before and after the Switch Point, changes were small and the relationship between oxy-Hb and deoxy-Hb was a combination of parallel and reciprocal fluctuations. A synchronized, parallel and short-lived change in oxy-Hb and deoxy-Hb is a discrete event in the transition period between wakefulness and sleep. The concentration of these light absorbing molecules is abruptly set to a new level at sleep–wake transitions and probably reflects the different perfusion demands of these states.  2000 Elsevier Science B.V. All rights reserved. Themes: Other systems of the CNS Topics: Brain metabolism and blood flow Keywords: Near infrared spectroscopy; Sleep; Cerebral metabolism 1. Introduction mapping has shown that around the time of sleep onset, alpha frequency disappears and theta frequency increases New techniques for functional imaging of the brain have especially along the midline and in posterior regions [75]. great promise for elucidating basic neurobiological mecha- Increased delta and decreased beta activity as well as nisms (see [66]). While the process of waking activation increased EEG power have also been observed [4,44,52]. (see [54]) and sleep-related changes [1,3,4,10,22,24,33,36– Given these distinct EEG changes and the profound and 38,45,46,57,67] have received much attention, relatively global changes in consciousness and functional capacity in unexplored by neuroimaging techniques is the boundary the transition to sleep, it is somewhat surprising that the between wakefulness and sleep. Quantitative EEG brain reduction in cerebral metabolic rate and cerebral blood flow are small and gradual [16,32,46,57]. The characteriza- tion of the sleep onset process as incremental may be a 1 Published on the World Wide Web on 25 April 2000. function of the limitations of the techniques employed (see *Corresponding author. Tel.: 11-212-832-1544; fax: 11-212-650- [51,55]). To better understand the sleep state transition 5722. E-mail address: thrilla834@aol.com (A.J. Spielman) process the present study employed near infrared spec- 0006-8993 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993(00)02320-9