**Swaddling probably calms newborns through a combination of comforting body contact and fewer startle-driven movements.** The calming effect is supported by research; the precise contribution of each mechanism is less certain.

1. **It limits the movements that can wake or upset a baby.** Newborns have a strong Moro reflex: they suddenly throw their arms outward when startled, sometimes waking themselves and crying. A snug wrap contains those movements. The American Academy of Pediatrics describes this as one reason swaddling helps babies settle. [AAP explanation](https://www.healthychildren.org/English/ages-stages/baby/sleep/Pages/Good-Night-Sleep-Tight.aspx)

2. **It makes a startle less likely to become a full awakening.** In a 2002 study, Gerard and colleagues observed 26 infants during swaddled and unswaddled naps. Swaddling reduced startles and reduced the likelihood that a startle progressed to waking during quiet sleep. This provides a physiological explanation for some of the calming you see. These infants were about 80 days old, so the study does not establish the exact response in a four-day-old. [Study in *Pediatrics*](https://pubmed.ncbi.nlm.nih.gov/12456937/)

3. **The surrounding contact may feel familiar and soothing.** Swaddling provides bodily boundaries resembling the confined environment of the womb. The AAP offers this as an explanation for its soothing effect. “It resembles the womb” is a plausible explanation, though the baby's subjective experience is difficult to establish experimentally. [AAP guidance](https://www.healthychildren.org/English/ages-stages/baby/diapers-clothing/Pages/Swaddling-Is-it-Safe.aspx)

The sleep effect has also been measured directly: a 2005 study of 16 infants aged 6–16 weeks found that swaddling increased sleep efficiency and reduced spontaneous awakenings. That supports your observation, while leaving some uncertainty about exactly why it works. [Franco and colleagues, *Pediatrics*](https://pubmed.ncbi.nlm.nih.gov/15867039/)

**For your four-day-old, aim for snug, with room for breathing and hip movement.** Increasing tightness can affect breathing: a study found that greater swaddle pressure increased respiratory rate during quiet sleep. [Gerard and colleagues, 2002](https://pubmed.ncbi.nlm.nih.gov/12219062/)

The AAP recommends allowing two or three fingers between the wrap and the chest, leaving the legs free to bend up and outward, and avoiding overheating. For sleep, always place your baby on their back on a firm, flat surface; keep the head uncovered, use an unweighted swaddle, and stop swaddling at the first signs of trying to roll. Swaddling has not been shown to prevent SIDS. [AAP safe-swaddling guidance](https://www.healthychildren.org/English/ages-stages/baby/diapers-clothing/Pages/Swaddling-Is-it-Safe.aspx)