Fig. 5.

Autonomic control during the restraint session was altered by day length. Autonomic blockade studies were conducted as above, but hamsters were restrained immediately after injections. Short photoperiods enhanced the withdrawal of parasympathetic tone and the increase in sympathetic control over the heart. Data are presented as mean (milliseconds of heart period) ± SEM, and mean differences were considered statistically significant when P < 0.05; *, significantly different from long days.