OBJECTIVES:Despite a relatively large body of literature describing the characteristics of sleep spindles and K-complexes in young adults, relatively little research has been conducted in older individuals. The general consensus from the few studies that have addressed this issue is that there is a progressive decrease in the number of spindles and K-complexes with age, although there is large intra-individual variation. Whether or not these changes are an inevitable consequence of the aging process can be addressed by studying healthy older adults who provide an example of the effects of age independently from those of disease. METHODS:Fourteen young adults (mean age=21.4+/-2.5 years) and 20 older adults (mean age=75.5+/-6.3 years) participated in the study. All subjects were neurologically and medically healthy and were not taking any medications with a known effect on the central nervous system or sleep. For each subject, a number of characteristics were determined including the number, density (SS/min), amplitude and frequency of all spindles as well as the number and density of K-complexes (KC/min). RESULTS:Spindle number, density and duration as well as K-complex number and density were all significantly lower in the elderly compared to the young adults. The EEG frequency within the spindles was significantly higher in the elderly, although the absolute difference was less than 0.5 Hz. Multiple regression analysis indicated that spindle duration and K-complex density were able to predict over 90% of the variance in age. CONCLUSIONS:The age-related decrease in sleep spindle and K-complex density is consistent with previous reports and may be interpreted as an age-related alteration of thalamocortical regulatory mechanisms.