Melatonin suppression in human subjects by bright and dim light in antarctica: time and season-dependent effects.

Full-spectrum light, of sufficiently high intensity, will suppress the secretion of melatonin at night in humans. Individual sensitivity to such suppression is variable, and the factors determining such sensitivity are largely unknown. By analogy with animal work previous short or long-term exposure to different light intensities may be an important determinant. We exploited the Antarctic environment to investigate these possibilities. Groups of healthy men, living on the British Antarctic Survey Base at Halley (75 degrees South) were exposed to dim (range 290-310 lux) and bright (range 2100-2300 lux) light either from 01.00-02.00 h or 05.00-0.600 h, both in winter and in summer. Plasma melatonin concentrations were determined by radioimmunoassay in serial blood samples taken before, during and after light treatment, and in control (darkness) conditions. Light suppression of melatonin was more effective in the latter part of the night in winter and this was particularly well-differentiated for dim light.