Circadian rhythms are patterns in behavioural, physiological and many others parameters that recur approximately every twenty four hours. Dysfunction of the circadian system is being linked to a number of common illnesses. The psychiatric condition Attention Deficit/Hyperactivity Disorder (ADHD) can be characterised by an early onset of sleep problems and breakdown of stable circadian rhythms which recent studies have shown. The drug atomoxetine used in this research project is used in the treatment of ADHD. It acts primarily through the noradrenergic system and is not a psychostimulant like the other mainstays of ADHD management such as Ritalin and Amphetamine. The main aims of this research project are to examine if acute treatments with atomoxetine produces alterations in the circadian rhythms in mice. In addition to examine whether treatment with these drugs alters cellular activation in brain areas implicated in the circadian timekeeping system. The effects of atomoxetine on locomotor circadian rhythms in mice were examined under free-running conditions (to look at the endogenous circadian clock without influence of environmental factors like light). Results have shown that atomoxetine produces a novel resetting of circadian time in mice, producing significant phase delays in animals in constant light and phase advances in animals in constant dark. Atomoxetine also altered the expression of c-Fos and clock genes in the master circadian clock. As there is evidence that ADHD might be associated with misalignment of the circadian clock with the environment, Atomoxetine might ameliorate some of the symptoms of ADHD by resetting the clock.