Background: Breast cancer is one of the most common female malignant cancers. Biorhythm disorder largely increases the risk of breast cancer. We aimed to investigate the biological functions and molecular mechanisms of circadian gene TIMELESS circadian regulator (TIM) in estrogen receptor (ER)-positive breast cancer and provide a new therapeutic target for breast cancer patients. Methods: The concentration of sphingosine 1-phosphate (S1P) was detected by high-performance liquid chromatography-tandem mass spectrometry. The Oxygen consumption rate was measured by XF96 metabolic flux analyzer. The interaction between TIM and Specificity protein 1 (Sp1) was confirmed by Co-immunoprecipitation. Chromatin immunoprecipitation and luciferase report assay were performed to demonstrate whether SP1 binds on the Alkaline Ceramidase 2 (ACER2) promotor. Findings: The expression of TIM was elevated in breast cancer, and high expression of TIM in cancer tissues was associated with poor prognosis, especially in the ER-positive breast cancer patients. In addition, we found that TIM promoted cell proliferation and enhanced mitochondrial respiration. TIM interacted with Sp1 which contributes to upregulate the expression of ACER2. Moreover, ACER2 is responsible for TIM-mediated promotive effects of cell growth and mitochondrial respiration. Interpretation: Our research unveiled a novel function of TIM in sphingolipid metabolism through interaction with Sp1. It provides a new theoretical explanation for the pathogenesis of breast cancer, and targeting TIM may serve as a potential therapeutic target for ER-positive breast cancer. Funding Statement: This study was supported by the National Natural Science Foundation of China (No. 81672358, No. 81802890, No. 81172505, No. 81302302). Declaration of Interests: The authors declare no conflict of interest. Ethics Approval Statement: The SHPD001 and SHPD002 trials were approved by the Independent Ethical Committee of Renji Hospital, School of Medicine, Shanghai Jiao Tong University. Before enrollment, all patients signed written informed consent. All animal experiments were approved by the Institutional Animal Care and Use Committee of East China Normal University.