**Swaddling probably calms babies through a combination of comforting body contact and limiting movements that can startle them awake.** The soothing effect is supported by research; the exact mechanism is less certain.

Two explanations are particularly relevant:

- **It limits sudden arm movements and their progression into waking.** Young infants make involuntary startles during sleep. Swaddling contains those movements, which may help prevent a brief startle from becoming a full awakening with crying. In a study of 26 infants, swaddling reduced startles and reduced the likelihood that a startle progressed to full arousal during quiet sleep. Those babies were about 3 months old, so this is evidence for the mechanism, rather than a direct study of 4-day-olds. [Gerard, Harris & Thach, *Pediatrics*, 2002](https://pubmed.ncbi.nlm.nih.gov/12456937/).

- **It provides a familiar feeling of containment.** A newborn has just moved from a confined environment into one where their limbs can move freely. A snug wrap provides continuous contact and boundaries around the body. The American Academy of Pediatrics describes this as resembling the womb and helping soothe newborns. That is a plausible explanation, but “it reminds them of the womb” should not be treated as a proven account of what babies experience. [AAP guidance](https://www.healthychildren.org/english/ages-stages/baby/diapers-clothing/pages/swaddling-is-it-safe.aspx).

Research also shows an effect on sleep itself: a 2022 systematic review found that swaddling tended to increase quiet sleep and reduce transitions between sleep states. However, only six studies met its criteria, and swaddling methods varied, so the evidence cannot tell us precisely why it works for your particular newborn. [*The effect of swaddling on infant sleep and arousal*, 2022](https://www.frontiersin.org/journals/pediatrics/articles/10.3389/fped.2022.1000180/full).

**“Snug” is the aim; tighter is not necessarily better.** For your 4-day-old, the AAP recommends:

- Enough room for **two or three fingers between the chest and wrap**, allowing unrestricted breathing.
- Loose wrapping around the hips and legs so they can bend up and outward.
- Sleeping **on their back, on a firm, flat surface**, with the head uncovered and no loose bedding.
- Avoiding overheating and weighted swaddles, and stopping swaddling as soon as the baby shows signs of trying to roll.

These precautions matter because excessive tightness can restrict breathing or harm hip development, and swaddling does not prevent SIDS. [AAP safe-swaddling guidance](https://www.healthychildren.org/english/ages-stages/baby/diapers-clothing/pages/swaddling-is-it-safe.aspx).