The effect of bright light exposure on pupillary fluctuations in healthy subjects

Background: Light therapy is thought to be the first choice treatment of winter depression. However, its way of action is poorly understood. In order to find a solid effect of bright artificial light, we studied its possible alerting action through the spontaneous fluctuations of the pupil, considered to be an objective measurement of vigilance. Methods: Pupillary fluctuations of 10 healthy subjects (mean age: 22±1 S.D. years) were measured for 60 s before and 15 min after 0.5 h, 10 000-lux light exposure. The cumulative change in pupil size, characterised by the pupillary unrest index (PUI) decreased at each subject, and this decrease was in average 35±4.4% S.E.M. The average pupillary diameters were unchanged (101±2.2% S.E.M.). This analysis revealed that the slow components of the pupillary fluctuations also decreased considerably. Limitations: There was no dim light or other placebo control of the light treatment. Conclusions: Bright light exposure significantly influenced the pupillary fluctuations. We suppose that bright light exposure increases the level of alertness, and this could be a possible way by which bright artificial light exerts a beneficial effect also in affective disorders.