The physiology of sleep and the impact of ageing

Abstract Sleep is an essential part of life. It is not a passive state of unconsciousness, but rather a dynamic brain processthat is the result of the interaction between two largely independent basic mechanisms: the circadian rhythm and the homeostatic drive to sleep. Its exact role has yet to be elucidated but it is known that sleep plays an important role in the restoration of physical and mental functioning. Sleep can be described both quantitatively and qualitatively, subjectively and objectively. Recent research hasled to a substantially improved understanding of both normal and altered sleep patterns, and their impact on health. A large diversity of factors has been described that can either favourably or adversely influence sleep, including sleep deprivation, sleep fragmentation, medical conditions and external stimuli. Normal sleep patterns vary significantly across the life-course, with more disrupted and less efficient sleep seeming to occur in the elderly. Sleep disorders are frequent and diverse in their origin, and commonly result in excessive daytime somnolence, which can have a major impact on daytime performance and safety in the individual. Also, chronic sleep disturbance is associated with increased morbidity and mortality. It is thus important that the quality of sleep is maintained, and situations and conditions known to have a negative impact on sleep initiation and maintenance should be prevented or treated.