FIGURE 4.

Key variables in circadian biology. A: circadian phase diagram showing key terms related to oscillatory variables, including zenith/peak (the highest value of an oscillating variable), nadir/trough (the lowest value of an oscillating variable), amplitude (the distance from peak to trough value of an oscillating variable), and phase shift (horizontal translation of the circadian oscillation with respect to time). B: example actograms showing an intervention with stable active phase/inactive phase rhythm entrained by a light-dark cycle (top) and progressive shifting (described by the parameter tau) of the active phase/inactive phase with respect to calendar time in a subject with an endogenous circadian period slightly greater than 24 h subjected to a constant routine without light/dark cues (bottom). Figure was created with BioRender.com, with permission.