So, quoting from the mental health guidelines - this document refers to people under stress making decisions about overnights and goes on to explain why they might not be a good solution - the following quoted from the report explains the situation generally NOT just in relation to divorce/separation, which is covered later in the guidelines more explicitly (NB I might be wrong, but it seems to me to be pretty obviously to be the case):
"i. despite the best of intentions, the absence of a primary caregiver when support is needed during the night is experienced by the infant and young child as frightening and inexplicable, because they don’t yet have the skills including language to understand it,
ii. an infant or young child does not have an understanding of time and hence absence or waiting may be experienced as a major loss,
iii. the care that is available, while thoughtful and well-intentioned, may be experienced by the infant or young child as unpredictable simply because it is different and therefore leads to anxiety,
iv. the significance of an infant or young child’s distress cues may be difficult to recognise by another carer, and if recognised, may need to be responded to by returning the infant to the care of the primary caregiver, which may not be possible if not thought about prior to the arrangement. When considering separating an infant or young child from their main caregiver overnight, some important factors that impact on infant development need to be considered. These factors include secure attachment, breastfeeding and stress
These are explanations of general issues too:
(i) Change
Change in our daily lives can be stressful for all of us and the younger the child the more likely that change is to be stressful. Very young infants can experience even small changes in their day as stressful such as a nappy change or bath (Perry et al, 1998). Similarly, parents and caregivers are aware of the stress experienced by even two-year-olds as a result of changes such as moving a young child from a cot to a bed or a parent returning to work. Young children in long day care and family day care have shown persistent stress reactions even when the group is small (family day care) and the carers are warm and responsive. Negative reactions are more prevalent when the care is controlling. Where possible the infant or young child’s primary caregiver supports the child through changes, ensures as far as possible that the change takes place at the child’s pace and not too many changes occur at once. This helps the child to build the resilience that will be needed to cope with change throughout life.
(ii) Night time fears
Night times are more stressful than daytimes for infants and children and for many adults as well (Dewar, 2014). When night time sleep problems occur, fears related to separation can arise in older infants and young children who are most likely to need the support of their primary caregiver in their familiar environment. ......Again, ideally, care would take place in the infant or young child’s familiar surroundings with the carers moving out for the period, rather than the child. This is of often very difficult in practice although some parents manage it for the benefit of their children
These covers how you can start to make overnights work (and is relevant to @blaablaablaa's post) but NB there is more in the documents and they are very easy to read and full of info which will help people make informed choices:
5. Whenever undertaken, overnight time away from the primary caregiver is best with a caregiver who is already a source of security and comfort to the infant or young child. In separated families, this level of security is optimally established and maintained over the first few years via regular, 2 to 3 times per week daytime caregiving-based contact that supports the baby’s routines. By the time the child is at least in part able to self-soothe, consistently turns to the other parent for comfort, and is less reliant on the primary attachment relationship to co-regulate his or her stress states, then the child is more likely to manage a gradual introduction to a well-supported overnight arrangement between cooperative parents.
6. Care should be taken not to fragment an infant’s schedule, for example, with long day care plus frequent visits with the other parent. When practical, and when the other parent is already a source of comfort and security to the infant, day-time care by the other parent should be prioritised above time spent in group day care.
7. Generally, in the third to fourth year, when developmental, emotional, parenting and practical conditions are supportive, regular overnight care with the other parent may be gradually phased in, at low frequencies, and always with monitoring of the child’s response.
8. Finally, in all scenarios, priority should be given to the child’s emotional security which should be carefully monitored and responded to sensitively and thoughtfully. When an infant under three has to be away from his or her primary caregiver overnight, regardless of whether the parents are separated or not, and assuming that parents are able to work cooperatively, the following strategies are recommended
As I said, both docs are clearly written.