identify a visual stimulus if you are tired. Likewise, manual dexterity, grip strength, and tapping, which assess motor ability, are susceptible to our circadian rhythm, with a low point in the morning and peak in the late evening. Bratzke wished to localize the time-of-day effects on processes that occur during the central decisional stage, such as decision-making. Along with his colleagues, he hypothesized that reaction time performance decrements during the night might be associated with slowing of this central processing stage. They refer to this as the central-slowing hypothesis. Bratzke observed six subjects during 28 hours of constant wakefulness. During this time, the subjects performed various cognitive tasks every two hours. Bratzke used a dual-task performance test (psychological refractory period paradigm) to assess the duration of the central processing stage with two stimuli introduced at different times. The results provided evidence for a circadian modulation on reaction time with a constant decrement in the late evening while reaching it's lowest point in the early morning. Importantly, dual-task interference increased in the night and early morning as well. Thus, central processing slowed down during the night, a result that supports the central-slowing hypothesis. It is important to note that Bratzke monitored circadian phases through salivary melatonin concentration and body temperature. These biological markers of circadian phases corresponded with the decrement in reaction time, telling us that sleep deprivation was not the only moderator in the performance decrement. These results build upon a recent study showing that even the well-practiced simple task