Getting to sleep during late pregnancy often seems like an exercise in futility. Since I knew that shut-eye would simply not happen, I anticipated going to bed more for a break from the trials of managing my life and bump. Adding to my anxiety was the idea that the onset of labor often occurs when the body is gearing up for sleep (ha!). It never happened for me, but it does for a lot of moms-to-be, so the question remains: does labor happen more at night?
The short answer is yes. In 2015, the Centers for Disease Control and Prevention (CDC) published data that showed the majority of births happened in the morning or midday hours, meaning many women began active labor when the moon was out. Melatonin, the sleep hormone, starts ramping up its production as sunlight decreases and builds up until bedtime when our bodies are ready to sleep, but in late-term pregnancies, this potent hormone does some interesting stuff that might bring your little bundle into the world before tomorrow's lunch.
"If it’s a natural labor, people do seem to go into labor in the middle of the night," says OB-GYN Dr. Christine Greves in an interview with Romper. "I tell my patients you never know when labor is going to happen and studies have demonstrated that if it’s natural, there’s an increased chance of it happening at night."
In a study conducted by Brigham and Women's Hospital in Boston, researchers found that late-term pregnant women exposed to light frequencies that mimic daylight had fewer uterine contractions than those women who were exposed to nighttime light frequencies and the results were likely triggered by melatonin receptors in the uterus. Everyone depends on melatonin, the sleep hormone, to get some Zs at night, but it seems these particular receptors are only present during pregnancy and interact with another common hormone, oxytocin, to stimulate contractions.
Now that you know you're more likely to start your labor after the sun goes down, does this newish knowledge have any healthcare applications? Turns out, it does. Researchers at Brigham & Women's Hospital in Boston hope that further study could produce a non-invasive means of treating pre-term labor using light-based therapy or a practice of adding melatonin to the labor induction protocol.
Interestingly, another study in the obstetrical journal Reproductive Sciences says this phenomenon implies that melatonin is a key ingredient in starting term pregnancies because the nightly onset of melatonin does not induce labor in women who are pre-term. "This is clearly not due to the absence of the hormone since human melatonin secretory rhythms continue throughout pregnancy and indeed melatonin levels increase in amplitude during late pregnancy," the report noted, because it's likely that the uterine receptors aren't quite as receptive until late pregnancy. Both hormones work synergistically, which is demonstrated when a chunk of uterus is introduced to the chemicals in a petri dish. First, the muscle doesn't respond with a small amount of oxytocin in the dish, but contracts once melatonin is added. Some health experts advocate letting women labor with low lighting or using "blue blocker" glasses to filter out lighting that mimics daylight to maintain the pace of labor.
This also means, of course, that you should be prepared.
"At night, the last thing you’re going to remember is to pack your items you really care about," Dr. Greves says. "Keep your bag in the car, ready to go at all times once you're 35 to 36 weeks, and have a plan in action for people to assist with that, especially if you have other children."
With all that in mind, be prepared to get even less sleep at the end of your pregnancy now that you know when labor is most likely to strike.