The function of BHLHE40 (basic helix-loop-helix family member e40, Ensembl gene identifier ENSG00000134107) is as follows. Transcriptional repressor involved in the regulation of the circadian rhythm by negatively regulating the activity of the clock genes and clock-controlled genes (PubMed:12397359, PubMed:18411297). Acts as the negative limb of a novel autoregulatory feedback loop (DEC loop) which differs from the one formed by the PER and CRY transcriptional repressors (PER/CRY loop) (PubMed:14672706). Both these loops are interlocked as it represses the expression of PER1/2 and in turn is repressed by PER1/2 and CRY1/2 (PubMed:15193144). Represses the activity of the circadian transcriptional activator: CLOCK-BMAL1|BMAL2 heterodimer by competing for the binding to E-box elements (5'-CACGTG- 3') found within the promoters of its target genes (PubMed:15560782). Negatively regulates its own expression and the expression of DBP and BHLHE41/DEC2 (PubMed:14672706). Acts as a corepressor of RXR and the RXR-LXR heterodimers and represses the ligand-induced RXRA and NR1H3/LXRA transactivation activity (PubMed:19786558). May be involved in the regulation of chondrocyte differentiation via the cAMP pathway (PubMed:19786558). Represses the transcription of NR0B2 and attenuates the transactivation of NR0B2 by the CLOCK-BMAL1 complex (PubMed:28797635). Drives the circadian rhythm of blood pressure through transcriptional repression of ATP1B1 in the cardiovascular system (PubMed:30012868). {ECO:0000269|PubMed:12397359, ECO:0000269|PubMed:14672706, ECO:0000269|PubMed:15193144, ECO:0000269|PubMed:15560782, ECO:0000269|PubMed:18411297, ECO:0000269|PubMed:19786558, ECO:0000269|PubMed:28797635, ECO:0000269|PubMed:30012868}.