This study explores the evolution of activity breaks in the nest cycle of annual eusocial bees, specifically focusing on the halictid species *Lasioglossum malachurum*. The authors present a model that explains the evolution of activity breaks by assuming differential mortality during active and inactive phases and a limited rate of larval development. The model predicts a systematic temporal structure of breaks at certain times in the season, which increase the fitness of the colony. The predicted pattern of these breaks aligns well with field data on the nest cycle of *L. malachurum*. The results challenge the assumption that only environmental factors govern the emergence of activity breaks, suggesting that the evolution of activity patterns is influenced by both endogenous and exogenous factors. The model's predictions are validated through numerical simulations and compared with field observations, showing good agreement. The study concludes that activity breaks are a counter-intuitive outcome of varying mortality rates that maximize the reproductive output of primitively eusocial nests.This study explores the evolution of activity breaks in the nest cycle of annual eusocial bees, specifically focusing on the halictid species *Lasioglossum malachurum*. The authors present a model that explains the evolution of activity breaks by assuming differential mortality during active and inactive phases and a limited rate of larval development. The model predicts a systematic temporal structure of breaks at certain times in the season, which increase the fitness of the colony. The predicted pattern of these breaks aligns well with field data on the nest cycle of *L. malachurum*. The results challenge the assumption that only environmental factors govern the emergence of activity breaks, suggesting that the evolution of activity patterns is influenced by both endogenous and exogenous factors. The model's predictions are validated through numerical simulations and compared with field observations, showing good agreement. The study concludes that activity breaks are a counter-intuitive outcome of varying mortality rates that maximize the reproductive output of primitively eusocial nests.