Biology of reproductionJun 1, 1991

Day length effects on womb activity and melatonin patterns during pregnancy in rhesus monkeys

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Updated Mar 28, 2026

Abstract

A shift in photoperiod altered the peak uterine activity in pregnant rhesus macaques from 2300 h to 1100 h after 7 days.

Myometrial contractile events displayed a clear nocturnal pattern, peaking at 2300 h and reaching a low at 0900 h.
Daytime plasma melatonin levels averaged 31 ± 6 pg/ml, while nocturnal levels peaked at 60 ± 6 pg/ml at 2400 h.
After reversing the photoperiod, the peak in uterine activity shifted to 1100 h and the nadir to 2200 h.
Melatonin concentrations increased from 22 ± 4 pg/ml during the light phase to a peak of 60 ± 5 pg/ml at 1300 h after the photoperiod shift.
A stable phase relationship between myometrial activity and melatonin rhythms was maintained following the photoperiod reversal.

Simplified

This study tested the hypothesis that photoperiod phase entrains the daily rhythms in myometrial contractility and melatonin in circulation in the pregnant rhesus monkey during late gestation. Six chronically catheterized rhesus macaques were maintained on a 12L: 12D cycle (lights-on, 0700 to 1900 h). Intrauterine pressure was continuously recorded throughout the study. At 120-135 days of gestation, blood samples were collected at 3-h intervals over a 24-h period beginning 2 h after lights-on at 0900 h. We observed a characteristic nocturnal uterine activity rhythm; contractile events peaked at 2300 h compared to the nadir at 0900 h (p less than 0.05, ANOVA). Daytime plasma melatonin levels averaged 31 +/- 6 pg/ml compared to the nocturnal peak of 60 +/- 6 pg/ml at 2400 h (p less than 0.05). Photoperiod was then shifted 11 h so that lights were on from 2000 to 0800 h. After 7 days, blood samples were again collected over 24 h, beginning 2 h after lights-on at 2200 h; this sampling protocol was repeated at weekly intervals until delivery. After 7 days of reversed photoperiod, the peak in the uterine activity shifted to 1100 h, whereas the nadir occurred at 2200 h (p less than 0.05). Melatonin concentrations increased from a mean of 22 +/- 4 pg/ml during the light phase to a nighttime peak of 60 +/- 5 pg/ml at 1300 h (p less than 0.01). A similar and stable phase relationship to lights-off was maintained for both the myometrial activity rhythm and the circadian melatonin rhythms after reversed photoperiod.(ABSTRACT TRUNCATED AT 250 WORDS)

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Day length effects on womb activity and melatonin patterns during pregnancy in rhesus monkeys

Abstract

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