Circadian oscillation of body temperature is a basic, evolutionarily conserved feature of mammalian biology(1). In addition, homeostatic pathways allow organisms to protect their core temperatures in response to cold exposure(2). However, the mechanism responsible for coordinating daily body temperature rhythm and adaptability to environmental challenges is unknown. Here we show that the nuclear receptor Rev-erb alpha (also known as Nr1d1), a powerful transcriptional repressor, links circadian and thermogenic networks through the regulation of brown adipose tissue (BAT) function. Mice exposed to cold fare considerably better at 05:00 (Zeitgeber time 22) when Rev-erb alpha is barely expressed than at 17:00 (Zeitgeber time 10) when Rev-erb alpha is abundant. Deletion of Rev-erb alpha markedly improves cold tolerance at 17: 00, indicating that overcoming Rev-erb alpha-dependent repression is a fundamental feature of the thermogenic response to cold. Physiological induction of uncoupling protein 1 (Ucp1) by cold temperatures is preceded by rapid downregulation of Rev-erb alpha in BAT. Rev-erb alpha represses Ucp1 in a brown-adipose-cell-autonomous manner and BAT Ucp1 levels are high in Rev-erb alpha-null mice, even at thermoneutrality. Genetic loss of Rev-erb alpha also abolishes normal rhythms of body temperature and BAT activity. Thus, Rev-erb alpha acts as a thermogenic focal point required for establishing and maintaining body temperature rhythm in a manner that is adaptable to environmental demands.