Sleep is essential for infant growth, well-being, and particularly for cognitive functioning, as well as emotional and behaviour regulation (Chaput et al., 2016; Krause et al., 2017). Healthy sleep is generally defined by adequate duration, appropriate timing and the absence of sleep disturbance (Gruber et al., 2014). There is no definite consensus as to what constitutes sufficient sleep duration at different ages (Chaput et al., 2016), but the National Sleep Foundation recommends that for every 24-hour cycle, infants between 4 and 11 months should sleep between 12 and 15 hours, with the normal range being 10–18 hours (Hirshkowitz et al., 2015). Studies indicate that 10–35% of parents report that their children have sleep problems such as short sleep duration, prolonged sleep onset, or frequent night-awaking (Byars, Yolton, Rausch, Lanphear, & Beebe, 2012; Galland et al., 2017; Hanafin, 2018; Hysing et al., 2014). Such sleep problems in young children not only affect the individual children themselves, but also have a profound effect on family functioning and parents’ well-being (Dennis & Ross, 2005; Martin, Hiscock, Hardy, Davey, & Wake, 2007), and often result in increased use of multiple health services (McCallum et al., 2011; Morris, James-Roberts, Sleep, & Gillham, 2001; Vandenplas, Hauser, & Salvatore, 2019).
Sleep–wake regulation and sleep states evolve rapidly during the first year of life, with continued maturation throughout childhood (McLaughlin Crabtree & Williams, 2009). In the first months of an infant’s life, the circadian rhythm is not yet established and sleep is normally characterised by short, fragmented periods of sleep with several awakenings (Davis, Parker, & Montgomery, 2004), due to the infant’s feeding needs. The circadian rhythm begins to emerge at around 10–12 weeks of age, marked by declines in night-time wakefulness over the first 6 months of life, but also a decrease in the total sleep duration from 16–17 hours in the newborn period (0–3 months) to 13–14 hours by 6 months of age (Bruni et al., 2014; Hirshkowitz et al., 2015). However, sleep patterns vary between individuals, and are explained by a complex interplay between genetic, environmental and social factors, including family routines, parenting practices and expectations (McLaughlin Crabtree & Williams, 2009). For most children, sleep problems are transient and likely to resolve naturally (Wolke, Bilgin, & Samara, 2017), but studies also suggest that sleep problems in infancy are related to later negative development, emotional and behavioural problems (Chaput et al., 2017; Hemmi, Wolke, & Schneider, 2011; Hysing, Sivertsen, Garthus-Niegel, & Eberhard-Gran, 2016; Mindell, Leichman, DuMond, & Sadeh, 2017; Seegers et al., 2016; Sivertsen et al., 2015; Smithson et al., 2018). The causal explanation for these associations are that sleep affects neurodevelopmental changes related to brain maturation, learning and memory consolidation (Born & Wilhelm, 2012) and that sleep may impact the brain circuits that underlie executive functions and higher-order systems involved in cognition (Bernier, Carlson, Bordeleau, & Carrier, 2010)
A recent systematic review of the relationship between sleep duration and health indicators in the early years (0–4 years) found that a lack of age-adequate sleep is prospectively associated with mental health problems such as anxiety, depression and poor emotional regulation. The evidence related to cognitive and motor development, however, was not clear (Chaput et al., 2017). Two large Norwegian birth cohort studies found an association between sleep problems in young children and social-emotional problems (Hysing et al., 2016; Sivertsen et al., 2015). Hysing and colleagues (2016) showed that less total sleep, prolonged sleep onset, and frequent night waking among 2,014 two-year-old children corresponded to greater risk of having concurrent social-emotional problems in toddlerhood. Similarly, Sivertsen and colleagues (2015) found that sleep problems among 18-month-old children (n = 32,662) were predictive of having emotional regulation difficulties both concurrently and at 5 years of age. There is a lack of studies reporting on sleep problems among infants below one year. A small longitudinal study of 117 infants and toddlers (ages 6 to 18 months) in the United States reported limited relationships between sleep consolidation and social-emotional and cognitive development (Mindell et al., 2017), and no significant relationships between any sleep variables and developmental outcomes (ages 3 to 13 months) on ASQ communication, fine and gross motor skills, problem-solving and personal-social skills (Mindell & Lee, 2015).
There is still a need for longitudinal studies to identify the long term outcome of children’s behavioral sleep problems in the first years of life. Thus, the aims of the present study were to (1) assess the prevalence of parent-reported sleep problems in a cohort of infants when they were 6, 12 and 24 months, (2) investigate the concurrent relationships between infants’ behavioral sleep problems and motor, communication, problem-solving, personal-social and social-emotional skills at 6, 12 and 24 months, and (3) to determine to what extent sleep problems at 6 months were related to changes in the developmental course from 12 to 24 months.