Transcription factor involved in the circadian clock. Binds to the promoter region of APRR1/TOC1 and TCP21/CHE to repress their transcription. Represses both CCA1 and itself. May recognize the promoter of JMJ14 to regulates its expression during the night in a circadian manner (PubMed:31429787). Homodimer or heterodimer with CCA1. Interacts with CCA1 (via internal domain); independently of photoperiod. Functions probably as part of a large complex. Interacts with CKB1 and CKB3. Interacts with LNK1 and LNK2 (PubMed:25012192). A number of isoforms are produced. According to EST sequences. Expressed in leaves, roots, stems, flowers and siliques. Circadian-regulation with peak levels occurring around 1 hour after dawn. Up-regulated by APRR1/TOC1 and transiently by light treatment. Down-regulated by APRR5, APRR7 and APRR9. Phosphorylated by CK2. Shorter circadian oscillations (PubMed:12015970, PubMed:19218364). The double mutant cca1 lhy accumulates higher levels of JMJ14 but lower levels of ATXR3/SDG2, and exhibits damped H3K4me3 levels near the transcription start sites of genic regions (PubMed:31429787). CCA1 and LHY are only partially redundant, but they bind to the same region of the promoters.