I remember the first year of my daughter's life as a strange mixture of joy and misery with the latter due to a chronic, grinding fatigue induced by bouts of prolonged sleeplessness. That chronic fatigue was occasionally punctuated by episodes of near delirious collapse due to the inability to catch up on lost sleep. Judging from reports of friends, acquaintances and the booming ‘getting baby to sleep' industry, my experience is not uncommon. Virtually every new parent has experienced the utter frustration of trying to get the baby to sleep...on their schedule. The baby, of course, does sleep. It has to if it is to survive. But it sleeps on its own schedule and that puts it at odds with its parents.
The tug-of-war between parents and babies over sleep schedules is an old story but advances in the science of sleep may throw some new light on this primeval phenomenon. The ‘getting baby to sleep' industry would like us to believe that the baby is a mere innocent who falls asleep whenever it can and wakes up whenever it needs to. The industry casts the mother in a similar innocent role: She just wants to get a good night's sleep while keeping the baby safe and warm? Is this story of tragic innocence believable or even helpful? Well, to some extent the story must be true; after all both the mother and the baby want the baby thrive but this story doesn't help us to solve the problem of getting the baby to sleep on parent's schedules.
The conflict over sleep schedules between baby and mother might be seen within the larger story derived from evolutionary game-theoretic models of parent-offspring conflict. In this story the mother uses signaling displays of her offspring (e.g. begging, vocalizations etc) to make investment decisions about offspring. To reach their own investment optima (to maximize their fitness by bringing offspring to reproductive maturity), parents need accurate information on need of offspring in order to effectively allocate resources to young. If a baby is to outcompete siblings and others for attention from mother it needs to signal to mother that is a worthwhile investment. Otherwise the mother will invest precious resources in a losing prospect thereby hurting survival prospects of herself and her other children. There is thus an onus on babies to accurately signal need and ‘viability', particularly in the crucial first year of life, in order to receive a consistent level of parental investment.
Given that mothers use signaling displays of offspring to make decisions about care, any physiologic system in the infant that produces these signaling displays would come under strong selective pressures (i.e. those juveniles not producing strong displays would receive less investment and therefore not survive to maturity or not reproduce at maturity). Since the infant spends up to 50% of its life asleep during the first year of life, and since at least one sleep state (REM) is associated with brain activation patterns that support signaling displays, there would be pressure to capitalize on REM's abilities to produce effective signaling displays even in the sleep state. We know that infants do in fact produce all kinds of signaling displays while in the REM state (coos, babbling, cries, smiles, gasping etc). Care received after signaling a need state, furthermore, would be as important for the infant as care received spontaneously from the mother, since responsive (rather than spontaneous) care taking would give the juvenile some control over its needs and its mother. Responsive care taking is more likely to occur after the infant signals with cries of distress or need which, in turn, are more likely when the infant awakes from REM rather than NREM sleep.
What all this means is that infant sleep, particularly REM sleep, has to perform signaling functions for the infant in addition to all the other functions sleep performs. Since these signaling functions include cries, wails and the like REM tends to awaken the infant with grating wails and cries-those things that reliably elicit responsive caretaking from the mother...and thus the mother cannot sleep when the infant sleeps.
The only solution to this dilemma is for the baby to have more than one or two ‘mothers'. That is the solution traditional societies invented. The extended family allows for multiple caretakers of a newborn and infant. In modern society however we have largely destroyed the extended family system. Parental collapse due to prolonged sleeplessness is only one of the consequences of this calamitous destruction.