Time allocation by lactating mammals is a reconciliation of often opposing nutritional and thermal demands of both the offspring and mother. Here we test the hypothesis that nest attendance patterns of lactating red squirrels (Tamiasciurus hudsonicus) vary with environmental and biological traits that relate to the thermoregulation of mothers and their offspring. We used temperature dataloggers to continuously record nest attendance and activity of free-ranging females with neonatal (n = 45) and preemergent (n = 53) litters. Lactating red squirrels concentrated activity out of the nest around the warmest parts of winter days and the coldest parts of summer days. Both younger and lighter litters had mothers that spent more time in the nest and shorter periods of time out of the nest. Females matched timing of activity within a day to the hourly air temperatures closest to their thermal neutral zone, serving to reduce individual and offspring thermoregulatory costs associated with activity. Nest attendance patterns appear to be constrained by the thermal, and possibly nutritional, requirements of the litter with females fine-tuning behavior to match constantly changing environmental and biological conditions, consistent with reduced energetic costs.