Differences over the life span in daily life-style regularity.

A diary-based instrument-the Social Rhythm Metric (SRM)-was used to assess the level of stability of daily social and behavioral rhythms in a group of 239 healthy subjects (112 male, 127 female) ranging in age from 20 to 89 years. Each subject completed the instrument for two consecutive weeks, which were averaged to yield one measure (SRM score) of life-style regularity [range 0 (least regular) to 7 (most regular)] and another of activity level index (ALI), corresponding to the number of (diary listed) activities done per week (max. = 119). SRM score increased reliably with age group at an average rate of 0.018 units per year. ALI showed an "inverted U"-shaped function with a maximum at about 50 years. SRM changes appeared not to be related to demographic differences between the age groups, although ALI differences may have been so related. No main effects or interactions were found with gender. Life-style regularity appears to increase over the life span in response to both biological and psychosocial changes and may represent an adaptation to age-related changes in the circadian system's sensitivity to entraining agents. Regular behavioral rhythms may be conducive to continued good health and well-being.