At nine months, infants who slept alone slept longer; this effect was largest for those who slept alone by four months, but also appears for babies who moved to their own room between four and nine months. Most notably, these differences were still present when the child was two and a half years old: children who slept alone by nine months slept forty-five minutes more during the night than those who were room sharing at nine months. Sleep is crucial for child brain development; it is not just a selfish parental indulgence. Of course, this may not be causal—maybe parents move their kids to
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