Actigraphy is an easily applied approach for assessing activity and circadian patterns in mother-infant dyads. However, timing and duration of actigraphic measurements can affect assessment accuracy. The purpose of this study was to determine the number of days of actigraphy data required to portray circadian rhythm in mothers and their young infants. Continuous actigraphy monitoring was performed in 20 mother-infant pairs over a 4-day period. Cycle mesor, amplitude, acrophase, and R2 were calculated and compared using from 1 to 4 days of data. Parameters based on 4 days of data were correlated with parameters derived from 1 to 3 days of data. There were no differences among mother or infant cosinor parameters except infant acrophase, which stabilized after > or = 2 days of data. Acceptable reliability (r > .80) was achieved with > or = 2 days of data. It was concluded that a recording period of 2 days adequately depicted circadian rhythm of actigraphy in mothers and infants.