« Although longitudinal studies have shown that overall, sleep becomes less fragmented with age, around 9–12 months, increased nightwaking has been observed (Anders, 1979; Moore & Ucko, 1957; Scher, 1991). This trend has been linked to changes in the socio-emotional (Paret, 1983; Scher, 2001a), cognitive (Scher, 2005), and gross motor domains (Scher, 1996).
[…]
The present study provides a community-based sleep study of infants in Western Canada where none had previously existed. It was found that the majority of infants were reported by their parents to sleep ‘‘through the night’’ by the time they were 6 months (67%); still, at the latter part of the first year, one out of four infants experiences nightwaking regularly. This corresponds to reports obtained in numerous studies across cultural contexts (Anders, Halpern, & Hua, 1992; Morrell, 1999; Sadeh, Acebo, Seifer, Aytur, & Carskadon, 1995; Scher, 1991).
[…]
From a different angle, another age-related finding that is worth noting is the association between developmental achievements and sleep–wake regulation. The prevalence of sleep difficulties was positively linked to neuro-developmental achievements, so that among the infants who were most advanced (e.g., independently walking by the 10–12 month assessment), more sleeping difficulties were reported. This finding is consistent with the contention that at the end of the first year, sleep-related difficulties are common in infants with advanced gross motor achievements (Scher, 1996), and positive communicative skills (Scher, 2001b). »
J. Pediatr. Psychol. (2008) 33 (4): 396-405. doi: 10.1093/jpepsy/jsn013