Pregnancy Induces an Earlier Chronotype in Both Mice and Women.

Daily rhythms generated by endogenous circadian mechanisms and synchronized to the light-dark cycle have been implicated in the timing of birth in a wide variety of species. Although chronodisruption (e.g., shift work or clock gene mutations) is associated with poor reproductive outcomes, little is known about circadian timing during pregnancy. This study tested whether daily rhythms change during full-term pregnancies in mice and women. We compared running wheel activity continuously in both nonpregnant ( n = 14) and pregnant ( n = 13) 12- to 24-week-old C57BL/6NJ mice. We also monitored wrist actigraphy in women ( N = 39) for 2 weeks before conception and then throughout pregnancy and measured daily times of sleep onset. We found that on the third day of pregnancy, mice shift their activity to an earlier time compared with nonpregnant dams. Their time of daily activity onset was maximally advanced by almost 4 h around day 7 of pregnancy and then shifted back to the nonpregnant state approximately 1 week before delivery. Mice also showed reduced levels of locomotor activity during their last week of pregnancy. Similarly, in women, the timing of sleep onset was earlier during the first and second trimesters (gestational weeks 4-13 and 14-27) than before pregnancy and returned to the prepregnant state during the third trimester (weeks 28 until delivery). Women also showed reduced levels of locomotor activity throughout pregnancy. These results indicate that pregnancy induces changes in daily rhythms, altering both time of onset and amount of activity. These changes are conserved between mice and women.

Circadian modulation of memory and plasticity gene products in a diurnal species.

Cognition is modulated by circadian rhythms, in both nocturnal and diurnal species. Rhythms of clock gene expression occur in brain regions that are outside the master circadian oscillator of the suprachiasmatic nucleus and that control cognitive functions, perhaps by regulating the expression neural-plasticity genes such as brain derived neurotrophic factor (BDNF) and its high affinity receptor, tyrosine kinase B (TrkB). In the diurnal grass rat (Arvicanthis niloticus), the hippocampus shows rhythms of clock genes that are 180° out of phase with those of nocturnal rodents. Here, we examined the hypothesis that this reversal extends to the optimal phase for learning a hippocampal-dependent task and to the phase of hippocampal rhythms in BDNF/TrkB expression. We used the Morris water maze (MWM) to test for time of day differences in reference memory and monitored daily patterns of hippocampal BDNF/TrkB expression in grass rats. Grass rats showed superior long-term retention of the MWM, when the training and testing occurred during the day as compared to the night, at a time when nocturnal laboratory rats show superior retention; acquisition of the MWM was not affected by time of day. BDNF/TrkB expression was rhythmic in the hippocampus of grass rats, and the phase of the rhythms was reversed compared to that of nocturnal rodents. Our findings provide correlational evidence for the claim that the circadian regulation of cognition may involve rhythms of BDNF/TrkB expression in the hippocampus and that their phase may contribute to species differences in the optimal phase for learning.