Pink noise: Effect on complexity synchronization of brain activity and sleep consolidation Junhong Zhou a , Dongdong Liu b , Xin Li b , Jing Ma c , Jue Zhang a,b,n , Jing Fang a,b a Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, People’s Republic of China b College of Engineering, Peking University, Beijing 100871, People’s Republic of China c Department of pulmonary, Peking University First Hospital, Beijing 100034, People’s Republic of China article info Article history: Received 28 November 2011 Received in revised form 10 March 2012 Accepted 4 April 2012 Available online 25 April 2012 Keywords: Fractal dimension Noise exposure Sleep stability Cardiopulmonary coupling abstract In this study, we hypothesized that steady pink noise is able to change the complexity of brain activities into a characteristic level and it might have significant effect on improving sleep stability. First, we carried out the brain synchronization test in which electroencephalogram (EEG) signals of 6 subjects were recorded. The whole experiment procedure was divided into 5 blocks in the alternative feeding process of 10-min quiet and 10-min noise. After the complexity analysis of fractal dimension, we found that the complexity of the EEG signals decreased with the introduction of the pink noise exposure, showing the brain waves tended to synchronize with the pink noise induction to reach a low level. For the sleep quality experiment, 40 subjects were recruited the group of nocturnal sleep experiment and 10 participants were chosen for nap test. Each subjects slept for two consecutive experimental periods, of which one is pink noise exposed and the other is quiet. For both nocturnal sleep and nap tests, the results in the noise exposure group showed significant enhancement in the percentage of stable sleep time compared to the control group based on the analysis of electrocardio- graphy (ECG) signal with cardiopulmonary coupling approach. This study demonstrates that steady pink noise has significant effect on reducing brain wave complexity and inducing more stable sleep time to improve sleep quality of individuals. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction As known to all, brain activity has some specialized brain waves appearing with different frequencies at different states. For example, the delta wave, with a relatively low frequency and high oscillation amplitude, characterizes the slow wave sleep stage in non-rapid eye movement period when people sleep (Vincenzo and Stuart, 2010). Many researches tried to induce the brain wave with external stimuli, such as light and sound. In these studies, it has been proved that a simple single structured source is insufficient to change brain waves, according to the falsification of ‘binaural auditory’ (Stevens, 2003; Wahbeh, 2007) which was once announced to have ability to induce theta waves of brain (Foster, 1990). However, brain activities are complex and chaotic, so it might be possible for a noise source with complex structures to synchronize the brain waves so as to induce people into a specialized sleepy state. In nature world exists three basic kinds of noise, that is, white noise, pink noise (1/f noise) and brown noise. Pink noise, which randomly distributes in low frequency band with the spectral density S(f) proportional to 1/f g with the exponent g being 1 (Halley and Kunin, 1999), is thought to exist in a profusion of fields such as heart-rate fluctuation of human beings (Leon, 2001), quasar emissions (Dutta and Horn, 1981), human cognition (Weissman, 1988), DNA base sequence structure (Voss, 1992), river discharge (Mandelbrot and Wallis, 1968), and cellular automata (Christensen et al., 1968), etc. This universality of pink noise suggests that it is a general demonstration of complex systems instead of a consequence of special physical interactions (Meijer et al., 1981). Considering the marvelous phenomenon of pink noise, we hypothesized that it has the ability to change the complexity of brain activity and synchronize it into a characteristic state. Based on this hypothesis, we carried out a test of recording electro- encephalogram (EEG) signals from 6 subjects in afternoon nap. The fractal dimensions of EEG signals were analyzed as the parameter of complexity. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/yjtbi Journal of Theoretical Biology 0022-5193/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jtbi.2012.04.006 n Corresponding author at: Center for BioMed-X Research, Academy for Advanced Interdisciplinary Studies, Peking University, No.5 Yiheyuan Road, Haidian District, Beijing 100871, People’s Republic of China. Tel.: þ86 10 62755036; fax: þ86 10 62753562. E-mail address: zhangjuezjh@gmail.com (J. Zhang). Journal of Theoretical Biology 306 (2012) 68–72