This is a great article from Science Daily regarding parenting plans for infants and toddlers showing some evidence about young children needing to have a primary attachment that is secure with one parent and how overnights with this other parent may be disruptive to the attachment with the primary caregiver. This is an interesting read. Remember, that there is research in other areas that does not support these finding. Nonetheless, it is an interesting read.
In a new national study, University of Virginia researchers found that infants who spent at least one night per week away from their mothers had more insecure attachments to the mother compared to babies who had fewer overnights or saw their fathers only during the day.