Welcome to parenthood! For many of us, parenthood is like being air-dropped into a foreign land, where protohumans rule and communication is performed through cryptic screams and colorful fluids. And to top it off, in this new world, sleep is like gold: precious and rare. (Oh, so precious.)

Throughout human history, children were typically raised in large, extended families filled with aunts, uncles, grannies, grandpas and siblings. Adding another baby to the mix didn't really make a big dent.

Nowadays, though, many moms and dads are going about it alone. As a result, taking care of a newborn can be relentless. There are too few arms for rocking, too few chests for sleeping and too few hours in the day to stream The Great British Bake Off. At some point, many parents need the baby to sleep — alone and quietly — for a few hours.

And so, out of self-preservation, many of us turn to the common, albeit controversial, practice of sleep training, in hopes of coaxing the baby to sleep by herself. Some parents swear by it. They say it's the only way they and their babies got any sleep. Others parents say letting a baby cry is harmful.

What does the science say? Here we try to separate fiction from fact and offer a few reassuring tips for wary parents. Let's start with the basics.

Myth: Sleep training is synonymous with the "cry-it-out" method.

Fact: Researchers today are investigating a wide range of gentler sleep training approaches that can help.

The mommy blogs and parenting books often mix up sleep training with "cry it out," says Jodi Mindell, a psychologist at Children's Hospital of Philadelphia who has helped thousands of babies and parents get more sleep over the past 20 years. In fact, most of the time, it's not that.

"I think unfortunately sleep training has gotten a really bad rap because it's been equated with this moniker called 'cry it out,' " Mindell says.

Indeed, the cry-it-out approach does sound cruel to many parents. "You put your baby into their crib or their room, you close the door and you don't come back till the next day," Mindell says. "But that's not the reality of what we recommend or what parents typically do."

And it's not what scientists have been studying over the past 20 years. Cry-it-out is an old way of thinking, says Mindell, author of one of the most frequently cited studies on sleep training (and the popular book Sleeping Through The Night).

In today's scientific literature, the term "sleep training" is an umbrella term that refers to a spectrum of approaches to help babies learn to fall asleep by themselves. It includes much gentler methods than cry-it-out or the so-called Ferber method. For example, some sleep training starts off by having the parent sleep next to the baby's crib (a method called camping out) or simply involves educating parents about baby sleep.

"All these methods are lumped together in the scientific literature as 'sleep training,' " Mindell says.

In several studies, parents are taught a very gentle approach to sleep training. They are told to place the baby in the crib and then soothe him — by patting or rubbing his back — until he stops crying. The parent then leaves the room. If the baby begins crying, the parent is supposed to check in after waiting some amount of time. In one study, these types of gentle interventions reduced the percentage of parents reporting sleep problems five months later by about 30%.

Myth: There's a "right" amount of time to let your baby cry when you're trying to sleep train.

Fact: There's not a strict formula that works for every parent (or baby).

There isn't a magic number of minutes that works best for checking on a baby after you've put her down, Mindell says. It really depends on what parents feel comfortable with.

"Doesn't matter if you come back and check on the baby every 30 seconds or whether you come back every five minutes," she says. "If it's your first child you're going in every 20 seconds." But by the third, she jokes, 10 minutes of crying may not seem like a lot.

There is no scientific data showing that checking every three minutes or every 10 minutes is going to work faster or better than checking more often. There are about a dozen or so high-quality studies on sleep training. Each study tests a slightly different approach. And none really compares different methods. In many studies, multiple methods are combined. For example, parents are taught both how to sleep train and how to set up a good bedtime routine. So it's impossible to say one approach works better than the other, especially for every baby, Mindell says.

Instead of looking for a strict formula — such as checking every five minutes — parents should focus on finding what Mindell calls "the magic moment" — that is, the moment when the child can fall asleep independently without the parent in the room. For some children, more soothing or more check-ins may help bring forth the magic, and for other babies, less soothing, fewer check-ins may work better.

With my daughter, I finally figured out that one type of crying meant she needed some TLC, but another meant she wanted to be left alone.

Even having a good bedtime routine can make a difference. "I think education is key," Mindell says. "One study I just reviewed found that when new parents learn about how babies sleep, their newborns are more likely to be better sleepers at 3 and 6 months."

"So you just have figure out what works best for you, your family and the baby's temperament," she says.

Myth: It's not real sleep training if you don't hear tons of crying.

Fact: Gentler approaches work, too. And sometimes nothing works.

You don't have to hear tons of crying if you don't want, Mindell says.

The scientific literature suggests all the gentler approaches — such as camping out and parental education — can help most babies and parents get more sleep, at least for a few months. In 2006, Mindell reviewed 52 studies on various sleep training methods. And in 49 of the studies, sleep training decreased resistance to sleep at bedtime and night wakings, as reported by the parents.

There's a popular belief that "cry it out" is the fastest way to teach babies to sleep independently. But there's no evidence that's true, Mindell says.

"Parents are looking for like what's the most effective method," Mindell says. "But what that is depends on the parents and the baby. It's a personalized formula. There's no question about it."

And if nothing seems to work, don't push too hard. For about 20% of babies, sleep training just doesn't work, Mindell says.

"Your child may not be ready for sleep training, for whatever reason," she says. "Maybe they're too young, or they're going through separation anxiety, or there may be an underlying medical issue, such as reflux."

Myth: Once I sleep train my baby, I can expect her to sleep through the night, every night.

Fact: Most sleep training techniques help some parents, for some time, but they don't always stick.

Don't expect a miracle from any sleep training method, especially when it comes to long-term results.

None of the sleep training studies are large enough — or quantitative enough — to tell parents how much better a baby will sleep or how much less often that baby will wake up after trying a method, or how long the changes will last.

"I think that idea is a made-up fantasy," Mindell says. "It would be great if we could say exactly how much improvement you're going to see in your child, but any improvement is good. "

Even the old studies on cry-it-out warned readers that breakthrough crying sometimes occurred at night and that retraining was likely needed after a few months.

The vast majority of sleep training studies don't actually measure how much a baby sleeps or wakes up. But instead, they rely on parent reports to measure sleep improvements, which can be biased. For example, one of the high-quality studies found that a gentle sleep training method reduced the probability of parents reporting sleep problems by about 30% in their 1-year-old. But by the time those kids were 2 years old, the effect disappeared.

Another recent study found two kinds of sleep training helped babies sleep better — for a few months. It tried to compare two sleep training approaches: one where the parent gradually allows the baby to cry for longer periods of time and one where the parent shifts the baby's bedtime to a later time (the time he naturally falls asleep), and then the parent slowly moves the time up to the desired bedtime. The data suggest that both methods reduced the time it takes for a baby to fall asleep at night and the number of times the baby wakes up at night.

But the study was quite small, just 43 infants. And the size of the effects varied greatly among the babies. So it's hard to say how much improvement is expected. After both methods, babies were still waking up, on average, one to two times a night, three months later.

Bottom line, don't expect a miracle, especially when it comes to long-term results. Even if the training has worked for your baby, the effect will likely wear off, you might be back to square one, and some parents choose to redo the training.

Myth: Sleep training (or NOT sleep training) my children could harm them in the long term.

Fact: There's no data to show either choice hurts your child in the long-run.

Some parents worry sleep training could be harmful long-term. Or that not doing it could set up their kids for problems later on.

The science doesn't support either of these fears, says Dr. Harriet Hiscock, a pediatrician at the Royal Children's Hospital in Melbourne, Australia, who has authored some of the best studies on the topic.

In particular, Hiscock led one of the few long-term studies on the topic. It's a randomized controlled trial — the gold standard in medical science — with more than 200 families. Blogs and parenting books often cite the study as "proof" that the cry-it-out method doesn't harm children. But if you look closely, you quickly see that the study doesn't actually test "cry it out." Instead, it tests two other gentler methods, including the camping out method.

"It's not shut the door on the child and leave," Hiscock says.

In the study, families were either taught a gentle sleep training method or given regular pediatric care. Then Hiscock and colleagues checked up on the families five years later to see if the sleep training had any detrimental effects on the children's emotional health or their relationship with their parents. The researchers also measured the children's stress levels and accessed their sleep habits.

In the end, Hiscock and her colleagues couldn't find any long-term difference between the children who had been sleep trained as babies and those who hadn't. "We concluded that there were no harmful effects on children's behavior, sleep, or the parent-child relationship," Hiscock says.

In other words, the gentle sleep training didn't make a lick of difference — bad or good — by the time kids reached about age 6. For this reason, Hiscock says parents shouldn't feel pressure to sleep train, or not to sleep train a baby.

"I just think it's really important to not make parents feel guilty about their choice [on sleep training]," Hiscock says. "We need to show them scientific evidence, and then let them make up their own minds."

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