Annual variation in attentional response after Methylphenidate treatment

Prevalence rates of Attention-Deficit/Hyperactivity Disorder (ADHD) differ with geographical areas varying in sunlight intensity. We here studied 1) annual variation in inattention at treatment initiation; 2) annual variation in response to ADHD treatment [Methylphenidate (MPH)] by day of treatment initiation; and 3) dose-dependency. We predicted least baseline inattention during a period of high sunlight intensity implying more room for improvement (i.e., a better treatment response) when sunlight intensity is low. These hypotheses were not confirmed. High dose treated patients however had significantly better attention than low dosed treated patients, only when treated in the period from winter to summer solstice. Change in solar irradiance (SI) during treatment period was negatively related to attentional improvement. The above described findings were primarily found in inattention ratings and replicated in omission errors on a Continuous Performance Task. Daylight and inattention have been proposed to be related via mediation of the circadian system. One mechanism of MPH may be to enhance sensitivity to the diurnal entrainment to sunlight and the question can be raised whether appropriate lighting could potentiate the effects of stimulants.