In an earlier review, Ingram et al concluded that research on sleep and aging in animals was sparse and essentially in a formative stage. In the past several years, systematic research on age-related changes in sleep in animals has increased, along with a greater tendency to publish full-length reports with complete methodological details. Consequently, the status of research in the area may be upgraded to an intermediate level. Generally, recent findings confirm and extend earlier ones that a wide variety of age-related changes in sleep occur in several mammalian species. Inconsistencies still exist in regard to how selected parameters change in a particular species (for example, nonparadoxical sleep in aged cats). However, in contrast to the findings of Ingram et al, a larger number of age-related changes in sleep have been recently replicated in separate laboratories. Another area of congruence between earlier and more recent reports involves age-related alterations in circadian rhythms. Multiple changes in circadian function have been reported in many species, suggesting a widespread pattern of temporal disorganization in senescence. Among other changes, both the amplitude and the period of individual rhythms may be altered, often resulting in desynchronization both between rhythms and from environmental cycles. At present, no single animal model demonstrates the age-related changes that occur in human sleep. However, many of the age-related changes that humans exhibit in sleep and in circadian rhythms appear similar to those in other species. For example, rats show several changes in absolute levels of sleep, continuity of sleep, and circadian organization of sleep that approximate those in aged humans, while aged male cats show analogous changes in deep nonparadoxical sleep and in delta waves. These findings suggest that different species may be useful in modelling different aspects of sleep in aged humans. Relationships between sleep and memory in aged rats may also exhibit analogies to human aging. Deficits in paradoxical sleep, and more generally, in the continuity of sleep, have been related to cognitive deficits in aged humans. These similarities between rats and humans further enhance the potential usefulness of rats to model aspects of human aging. Many areas still require further study. The generality of age-related changes in sleep in animals cannot be determined until additional strains and species are evaluated. In addition, many sleep-related phenomena such as phasic events during paradoxical sleep have not yet been examined in old animals.(ABSTRACT TRUNCATED AT 400 WORDS)