Modulation of plasma melatonin concentrations by changes in posture

Abstract

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.

Human circadian melatonin rhythm phase delay during a fixed sleep–wake schedule interspersed with nights of sleep deprivation

“…Such effects have been reported in animal studies in which light exposure and physical activity were carefully controlled [41–43], and in blind individuals [44] and more recently also in sighted subjects [45]. Our data confirm previous findings that posture affects plasma melatonin concentrations with lower melatonin levels during supine positions [46, 47]. A standing position results in a redistribution of the blood onto the lower parts of the body, which includes the dependent arms and legs.…”

Influence of Light at Night on Melatonin Suppression in Children

“…It has been shown that posture can influence melatonin concentration in some studies (37,38) but not in other studies (39). Although the effect of posture on melatonin concentration remains controversial (40), illuminance at eye level varies depending on posture.…”

Eventual Entrainment of the Human Circadian Pacemaker by Melatonin is Independent of the Circadian Phase of Treatment Initiation: Clinical Implications

“…During each admission, no restrictions were placed on posture (although subjects were recumbent most of the time). We think that the effects of posture on melatonin levels remain unclear: 2 groups have reported a modest increase in melatonin levels when subjects change their posture from supine to standing (Deacon and Arendt, 1994;Nathan et al, 1998), another reported a modest decrease in the subjective day during a forced desynchrony protocol under dim light conditions (Shanahan and Czeisler, 2000), and a fourth group reported no effect (Voultsios et al, 1997). Subjects were not allowed to engage in physical exercise, but otherwise no restrictions were made on physical activity.…”