Along with swaddling, Dr. Karp says white noise is the key to the sleep puzzle. "In the womb, [babies] have a sound that is louder than a vacuum cleaner 24/7. So to be in a silent room is bizarre beyond belief for them," he says.
He adds there are two types of white noise: low and high pitches. High-pitched noises — like beepers, alarms, and sirens — are great for getting a baby's attention. But lower-pitched noises — like the rumble of a car, train, or plane — are more soothing and can lull you to sleep. "The sound in the womb is very low and rumbly because babies are underwater and water filters out the high-pitch sounds," Dr. Karp says. "Most of the machines, the apps, have the wrong sound. They have a shhhh, which is irritating as hell to parents and not helpful to a lot of babies too. Low pitch — like the sound of rain on the roof — that you play all night and at all naps encourages a baby to sleep. So it helps babies sleep in that fourth trimester [the first three months]."
Dr. Karp goes on to explain that if they are used to the white noise from birth, it will train them to put themselves back to sleep when they wake in the middle of the night. "At four months, when you stop swaddling and babies get nosy, they start waking up at night, and if they're in a totally quiet room, they call for your assistance. That's why everyone's doing sleep training at four months. But you don't have to do sleep training if you get them used to these cues earlier on — the swaddling and the low-pitch white noise CD. Then you wean the swaddling, and when your 4-month-old wakes up in the middle of the night, they hear the rrrrrrrr sound and think, 'Everything's fine, I can go back to sleep.' If they wake up in a quiet room, they scream for their mom because that's who was with them when they fell asleep originally."