Circadian latency variability of sympathetic skin responses

Sympathetic skin responses (SSRs) have been increasingly used as tests for autonomic function in the clinical setting. In spite of the known circadian rhythmicity of sympathetic function, however, normative studies have not addressed the possibility of circadian variability of SSR parameters. Ten normal volunteers (7 men, 3 women, aged 19 to 43) had SSR testing performed in the morning, at noon, and in the early evening, and response latencies and amplitudes were compared for the different day periods. Although amplitude values varied in a random fashion, regardless of the time of testing, there was a statistically significant variability in response latencies, which were, on the average, approximately 150 ms shorter in the morning trials, as compared to the early evening ones. This difference was statistically significant (ANOVA, p < 0.01). We propose that circadian variability of SSR latencies should be taken into account in normative studies of SSR parameters.