Using HF cells collected from 24 young men in both Friday and Monday trials, it was also found that there was a significant main effect of time on the expression of NR1D1, NR1D2, and PER3 in the morningness group but not in the eveningness group, indicating chronotype and social jetlag influence circadian clock gene expression [75]. This evidence concerns the gene NR1D2 and hydrops fetalis.