Clock time at which sustained daily activity begins, used as the phase marker for entrainment and phase-shift studies.
Use activity onset when the question is about the timing (phase) of the circadian clock rather than how much the animal moves. Onset is the most stable phase reference in nocturnal rodents and anchors phase-shift, jet-lag, and entrainment analyses. It is only meaningful once the onset-detection rule, bin size, and activity threshold are fixed before scoring.
| Primary value | Time of sustained activity onset relative to lights-off (ZT12) or, in DD, to subjective night (CT12) |
|---|---|
| Common units | Clock time, Zeitgeber time (ZT), or Circadian time (CT) |
| Compatible assays | Home-cage video tracking, running wheel, infrared beam-break, telemetry |
| Required boundary | Onset-detection rule, bin size, and activity threshold |
| Do not infer alone | Rhythm strength, period length, or total activity |
Bin activity (commonly 1, 5, or 10 min), then detect onset with a fixed rule such as a threshold crossing sustained for a minimum number of bins, applied consistently across animals. Onset is read most reliably from a double-plotted actogram, where it forms the leading edge of the active band.
A shifted onset indicates a phase change, not a change in clock speed or rhythm amplitude. Masking by light, noise, or feeding can move apparent onset without moving the underlying pacemaker, so onset under constant conditions is more interpretable than onset under a bright light-dark cycle.
Store animal ID, lighting condition (LD/DD/LL), bin size, onset rule and threshold, daily onset times, day index, and software or scorer version.
Endpoint pages should cite the method literature behind the scored value and keep high-specificity protocol claims qualified unless the source supports them.
Endpoint articles link to adjacent products, software workflows, and sibling endpoints where the connection is useful and already routable.
Video-based home-cage workflow that extracts onset, period, and amplitude.
How onset is read off a double-plotted actogram.
The period endpoint that onset drift reveals under constant darkness.
Compare onset with amplitude, period, and other circadian outputs.
