This study evaluated the putative association between fluctuations in preschool-aged children's motor activity level (AL) and fluctuations in the intensity of the earth's magnetic field, referred to as geomagnetic pulsations. A sample of 32 preschoolers wore two different types of activity monitors on their right ankles for five hours once a week for ten weeks. A measure of child's sleep was completed by each child's parent on the morning of each activity assessment. It was hypothesized that high-amplitude pulsations in the intensity of the earth's magnetic field would, by disturbing their sleep the night before, decrease children's AL. Sleep disturbance and AL measured on geomagnetically active days were contrasted with the same measures on geomagnetically quiet days. Three different thresholds for defining geomagnetically active days were evaluated. Regardless of threshold used, sleep disturbance was unrelated to differences in geomagnetic activity. However, when the highest threshold was used to define ageomagnetically active day and contrasted with the quietest day, AL was significantly lower on the geomagnetically active day. It is concluded that the evidence provides some tentative support for a link between geomagnetic disturbances, which are "rapidly varying perturbation to Earth's magnetic field" (National Academies, 2001), and activity level, but this link does not appear to be mediated by sleep disturbance.