Testosterone levels are subject to a circadian rhythm, peaking in the early morning hours and then falling by at least 43%. (2003)
From the publication:
Total testosterone
Each subject in both the old and young groups showed a marked diurnal variation in serum total testosterone, with a minimum decrease (peak to nadir) of 43% of the peak value. In each group, 50% of the subjects reached a nadir of < 10 nmol/l, while all subjects in both groups had a peak level of > 10 nmol/l. […] A significant difference between the young and older mean total testosterone was shown at three sampling times: 06·00, 07·00 and 07·30 h, when the young group showed a higher mean concentration at each of the three times. In both groups, the acrophase occurred at 07·30 h.
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Bioavailable testosterone
Both groups display a significant circadian rhythm. No significant difference was demonstrated in mesor or acrophase but a significant difference was shown in amplitude. The acrophase in both groups coincide at 07·24 h, similar to that for the total testosterone. Significant differences between the young and middle-aged groups in the mean bioavailable testosterone were seen at 04·30, 05·30, 06·00, 07·30 and 09·30 h.
Free testosterone
A highly significant circadian rhythm was observed in the young group and in the older group. As with the bioavailable testosterone, there was no significant difference between the two groups in mesor and acrophase but a significant difference was seen in amplitude. The acrophase was calculated as occurring at 07·18 h in the young group and 07·05 h in the older men. Significant differences in the mean free testosterone concentration were seen at 05·00, 05·30, 06·00, 06·30, 07·00, 07·30, 09·00, 09·30, 10·00 and 18·00 h.