Sex and hand differences in circadian wrist activity are independent from sex and hand differences in 2D:4D

Bilateral wrist activity of male and female college students was measured for three consecutive days. Digit ratio was obtained from photocopy measurements of the second and fourth fingers of each subject.Males had lower digit ratios with more pronounced differences on the right hand. Female acrophase occurred earlier than male acrophase. There was more activity in the right hand and right hand activity peaked before the left. Digit ratio was not correlated with any measure of wrist activity. An analysis of activity by age revealed that younger female students exhibited more male-like activity patterns.Sex and hand differences for digit ratio and acrophase replicated previous findings. The lack of correlation between digit ratio and patterns of wrist activity suggests that sexually dimorphic circadian activity develops independently from the mechanisms of hormone exposure that cause sex differences in digit ratio.The main circadian pacemaker responsible for maintaining the sleep/wake cycle in mammals is located in two bilaterally symmetrical suprachiasmatic nuclei (SCN) [1]. Each SCN comprises its own oscillator [2,3], but the two structures are thought to work in tandem to generate a single rhythm. However, when both sides of the body are monitored simultaneously (such as with wrist actigraphy), the rhythms for each side vary slightly [2] and the activity of the dominant hand has been shown to peak before the non-dominant hand [2,4]. There are also sexually dimorphic activity patterns, such that females reach acrophase earlier in the day than males [5] and female chronotype is considered to be morning-type while males are deemed evening-types [6,7]. Cerebral dominance may be readily assessed, but how patterns of differences in anatomical and functional asymmetries are correlated to cerebral dominance are not as straightforward [8].While sex, hand and handedness all appear to influence circadian regulation of activity patterns, their relative contributions remain unk