Research methods and neurophysiological mechanisms behind the alerting effects of daytime light exposure

An important aim of today's lighting research is defining and quantifying the optimal parameters of light for maximal alerting response. They would be useful in various clinical and non-clinical applications. The debate has covered the light intensity, the spectral distribution, the duration of the light exposure, and the circadian phase in which the light is administered. However, no consensus over the optimal parameters has been found. The thesis claims that the lack of consensus is due to three things. First, the terminology around alertness is confounding and alertness has not been defined properly. Second, the methods used to study light-induced alertness are not always optimal, because they do not consider the delicacy of the light stimulus nor are they based on the physiology of alertness. Third, the research has concentrated too much on effects instead of causes and therefore the mechanisms behind the alerting effects of light are not yet fully known. The thesis addresses these problems. It begins by discussing the physiology and the terminology behind the term "alertness". On the basis of the discussion, it defines alertness as an activation state of the brain that is induced by stimuli and reduced by the lack of them. The thesis continues with an evaluation of the methods used to study light-induced alertness. First the methods are assessed theoretically and after that the practical testing of heart rate, skin conductance, pupil size and subjective sleepiness is reported. Together, the evaluations show that the methods based on the autonomic nervous system activity are good for detecting the effects. For investigating the causes behind these effects monitoring the central nervous system should be used. Brain imaging is highly recommended. Further on the thesis discusses the neurophysiological mechanisms behind the light-induced alertness in daytime, which are less known than the night-time mechanisms. A new model of two separate pathways from the retina to the activation system is suggested. The new model links emotions to the alerting effects of light. The correlation of subjective mood and alertness is verified with a practical study. A test protocol to study the causal relationship of light, emotions and alertness objectively is also suggested. In addition to the objectives, a few interesting observations from the practical studies are reported. First, exposure to blue-enriched white light was shown to help students to retain their alertness during the natural dip time in the afternoon in the autumn compared to exposure to normal white light. Second, changes in the light exposure were shown to induce greater alerting effects than a constant light exposure. These observations contribute to the search for the optimal parameters of light exposure