The emerging discipline denominated "chronoecology of behaviour" is the result of the interaction between the work of behavioural ecologists and chronobiologists -- those who explain biological rhythms on the basis of experimental studies carried out in the laboratory. Since the beginning of this century, this interaction has generated interest in understanding how biological rhythms behave in organisms living in their natural habitat. This approach requires field studies of rest/activity rhythms in order to obtain detailed hour/hour and day/day information about the possible effects of the modulation of certain environmental variables, such as photoperiod, temperature and the lunar sky, which may synchronise and/or mask behavioural activity. This book looks at the behavioural variations in non-human primates and Homo sapiens to find out how the circadian rhythm modulates behaviour and in what way social interaction influences the rhythms of circadian activity. This book also analyses the interaction between the circadian and immune systems, and explores the signal transduction pathways which could participate in this dialogue. The precise knowledge of this interaction might be extremely useful for the understanding of diseases development such as cancer. Other chapters in this book assess the quality of sleep in children and how it affects the rhythmical functions of the body. The comprehensive coverage of this topic is presented together with a general perspective from a child, caregiver and healthcare provider.