Fig. 6.

Basal and stress-induced autonomic control. Data are plotted across a 2D autonomic space, with circles indicating basal autonomic tone and arrowheads indicating responses to acute restraint. Short photoperiods enhance both sympathetic and parasympathetic control over the heart under basal conditions. Following restraint stress, the withdrawal of parasympathetic control and the increase in sympathetic regulation are both enhanced by exposure to short day lengths. The x axis is sympathetic control over heart period (milliseconds), and the y axis indicates parasympathetic control over heart period.