It’s hard to find any literature on this, but as a private physician I would hesitate to allow it. Circadial rhythm is important to you and probably to your unborn baby in ways we haven’t discovered yet. Melatonin affects the chemistry of the pineal gland, which may allow you to sleep better but may be mucking up the works for your baby. Interestingly, the pineal gland is a remnant of a third eye for us–some amphibians have it under a membrane on the tops of their heads to pick up light and sense day and night, reacting with pigmentation and by regulating sleep-wake cycles. It may be harmless to experiment with someone already formed (you), but I’d be a little nervous exposing a developing baby to this. Especially in weeks 6-9 which are the most vulnerable weeks for developing abnormalities due to exposure to medicine and chemicals.