Modulation of plasma melatonin concentrations by changes in posture.

Posture change from a lying position to a standing position results in a decrease in plasma volume, which leads to an increase in plasma constituents, especially that of proteins and blood constituents bound to them. The aim of the present study was to investigate the physiological effects of postural changes on plasma nocturnal melatonin concentrations in healthy human volunteers. The study was divided into four stages. During stage one, subjects were seated from 21.00 hr to 01.00 hr. In stage two, subjects were lying at ground level from 21.00 hr to 01.00 hr. In stage three, subjects were is a sitting position from 2100 hr to 2300 hr and then in a standing position from 23.00 hr to 24.00 hr, and back to the sitting position from 24.00 hr to 01.00 hr. In the final stage, subjects were in a lying position from 21.00 hr to 23.00 hr and then in a standing position from 23.00 hr to 24.00 hr and back to the lying position from 24.00 hr to 01.00 hr. AUC analysis showed significant differences between sitting and lying positions (t=2.84; P<0.05; df=5), with higher melatonin levels associated with the sitting position (mean difference in peak concentration of 17.1 pg/ml). Furthermore a change in posture from the lying to the standing position produced a statistically significant increase in melatonin concentrations (final stage) (t=-3.37; P<0.05; df=5) (mean difference in peak concentration of 28.5 pg/ml). No differences were found with a change in posture from a sitting to a standing position. The hemoconcentration and hemodilution associated with posture changes may play a role in altering plasma protein bound hormones such as melatonin.