Small day-active endotherms living in hot environments maintain heat balance and avoid overheating by behavioral adjustments of their time budgets. We examined the temporal and spatial microstructure of surface activity bouts in the degu (Octodon degus) under maximal, summertime environmental heat load in a thermally complex habitat mosaic of sun and shade. The degu is the only native small mammal regularly active during the day in central Chile. We recorded thermal environmental data and activity of individuals with video cameras to quantify time spent in open sun as compared to shade. We predicted that individuals would adjust activity “microbouts” (successive momentary movements in and out of shade cover) in response to increasing heat load. As heat load increased toward end of morning, the proportion of total surface time spent in shade increased and approached 100%. Likewise, direct exposures to solar radiation decreased from almost 8 min in early morning to less than 1 min by late morning. Field measurements of body temperature remained within a 24-h range of 36–38°C (mean 36.7°C), suggesting an intolerance of increases above this range. The low heat tolerance of the degu seems to be a physiological limitation associated with its nocturnal family lineage. Because daytime activity is exceptional within this lineage, the degu has used behavioral adjustments to become the only regularly day-active native small mammal in its environment.