Examples of D3-determined activity onsets for (A) a hamster subjected to a 6-h phase advance of a 14 h:10 h LD cycle on the day marked by the arrow, and (B) a female hamster in DD. For the latter, in C, activity onset (advances are down, delays are up) is plotted in relation to day 1 of the estrous cycle (day of vaginal discharge) (*), and in D, the autocorrelation function (ACF) indicates peaks at lags of 4 days, indicating a 4-day periodicity; for a time series with duration of length N, the autocorrelation measure is valid for lags up to N/2.