As the U.S. population ages, families must assume increasing responsibility for the care of elderly members. Disruptive nocturnal behaviors (DNBs) of elders, such as complaining and demanding help, may result in interactions with caregivers that threaten such arrangements. This study aimed to quantify such interactions by cross-correlating motor activity that was simultaneously recorded from the elders and caregivers. Forty-four elder-caregiver pairs reporting DNBs simultaneously kept sleep logs and wore activity recorders for 6 to 8 days. Day and night activity data were analyzed separately, because circadian variations would otherwise have overshadowed the elder-caregiver covariations of interest. An autoregressive model was fitted to each day and night data segment, and the data-model differences were used to calculate a cross-correlation function. Maximum significant values of the cross-correlation functions (r _max) exceeded .300 in 10 pairs of subjects. The unprocessed motor activity of these pairs looked so similar that r _max was interpreted as a measure of the subjects' interactions. The r _max was significantly larger for nighttime activity, especially in pairs who shared the same bed. It was smaller in pairs whose elders had high depression scores and in those with Parkinson's disease or related disorders. It was not affected by the presence of dementia. Analysis of the lags corresponding to significant values of r _max showed that, in cohabiting pairs, it was mainly the elders who initiated interactions. The findings provide unique, objective evidence that the night is a time of special difficulty for many caregivers of older Americans.