The dysregulation of circadian genes like RBM3 and NR1D1 in both cluster headache and bipolar disorder (Costa et al., 2015) further strengthens the hypothesis of circadian system involvement, although subsequent studies, such as Fourier et al. (2018, 2021) showed mixed results regarding CLOCK and CRY1 gene expression. This evidence concerns the gene NR1D1 and bipolar disorder.