© 2018, Japanese Society of Sleep Research. Pregnant women have more sleep problems compared to non-pregnant women. Our objective was to investigate the differences in sleep patterns by age among Japanese women enrolled in the Japan Environment and Children’s Study during preconception and pregnancy. Questionnaires on sleep status were administered to pregnant women twice in the first, second and/or third trimesters. Wake-up time, bedtime, sleep duration and quality of sleep were analyzed in 103,099 respondents. Women younger than 20 years of age awoke and slept the latest in comparison to all other age groups during preconception and pregnancy. Sleep time of women younger than 20 years of age was the longest in comparison to other age groups. The wake-up time and bedtime of teenage mothers were delayed from preconception to second or third trimester, but they did not display any statistically significant changes in other age groups. Moreover, the younger the pregnant women were the more sleep trouble they had despite reportedly being tired, and felt more sleep-deprived despite having had enough sleep time in both second and third trimesters. The sleep state of pregnant women differed by age, with younger women experiencing more sleep trouble and feeling less rested compared to other groups. In future studies, the influences of sleep during pregnancy on the health and development of the children of these pregnant women will be clarified.