Best Sounds for Newborn Sleep, Ranked by the Evidence
Not every popular soothing sound has been studied the same way. Here's what's backed by research versus what's just widely used.
This is general information for a parenting/consumer audience, not medical advice. It doesn't replace guidance from your pediatrician, and every baby is different.
Strongest evidence: heartbeat and womb sound
Heartbeat sounds have specific research behind them, largely from NICU settings — studies combining a mother's heartbeat sound with white noise have reported improved cardiorespiratory and feeding outcomes in premature infants. Simulated intrauterine ("womb") sound has a similar evidence base, including documented use in reducing infant pain response during medical procedures. Real womb sound recordings run surprisingly loud — roughly 70–90 dB, well above typical NICU noise limits — and commercial "womb sound" products have been found to differ significantly from real recordings, so a synthesized "womb" tile is an approximation inspired by that research, not an authentic recording. Full breakdown.
Well-studied as a category: broadband noise
White noise generally — the category that includes rain, waves, fan, and static-like sounds — is the best-studied sound for infant sleep onset, going back to the 1990 founding trial. Individual variants (rain vs. fan vs. brown noise specifically) haven't each been separately tested; they're best understood as flavors of the same broadband-noise effect.
Popular, but not RCT-tested: shushing
"Shush" sounds are rooted in Dr. Harvey Karp's widely used "5 S's" soothing method. It's a well-known clinical and parenting technique, but we didn't find a controlled trial isolating the shushing sound itself the way the white-noise sleep-onset studies do. Treat it as expert-consensus and widely reported anecdotal effectiveness, not as proven in the same sense as the heartbeat or broadband-noise research.
Practical takeaway
If you want the option with the most direct research behind it, heartbeat or womb sound combined with a broadband noise (like rain or brown noise) mirrors what's actually been studied. But the honest answer is that which sound settles your particular baby is often trial and error — hushnap lets you mix one ambient sound with one soothing sound specifically so you can find that combination.
- Parga JJ, Daland R, Kesavan K, Macey PM, Zeltzer L, Harper RM. "A description of externally recorded womb sounds in human subjects during gestation." PLoS One, 2018.
- Spencer JAD, Moran DJ, Lee A, Talbert D. "White noise and sleep induction." Archives of Disease in Childhood, 1990.
- Harvey Karp, "The Happiest Baby on the Block" — originating source of the "5 S's" soothing method, including shushing.