That mothers shouldn’t co-sleep with infants. Every other primate I know of co-sleeps with their offspring. Until very recently every human mother co-slept with her infants, and in like half of the globe people still do. Many mothers find it incredibly psychologically stressful to sleep without their infant because our ancestors co-slept every generation for hundreds of thousands of years.
I would bet money that forcing infants to sleep alone has negative developmental effects.