Circadian variation of clock gene mRNA transcripts in human PBMCs. Transcript expression of hPer1, hPer2, hPer3, and hDec1 for each subject is shown with a solid line and the dual-harmonic regression with a dotted line. The y-axis represents the relative intensity of mRNA expression. The value of the lowest mRNA expression is designated 1, and the levels of mRNA expression at all other time points are calibrated to this value. Error bars indicate the standard deviation on the basis of the mRNA samples assayed in triplicates. The x-axis indicates the time, in hours, under the constant-routine procedure. In the melatonin (MLT) panel, the y-axis represents plasma melatonin concentration (picograms per milligram) in each subject. For the purposes of illustration, times where subjects are habitually asleep are projected as open rectangles (□).