Purpose <p>Sleep problems and behavioural challenges are examined rarely in very young autistic children. We investigated sleep in 173 autistic children, close to their autism diagnosis (<i>M</i>_<i>age</i> = 2.49 years at diagnosis), focussing on sleep’s relationship with daytime behaviour and vice versa, and examining if there were specific sleep problem-behaviour relationships in this very young cohort.</p> Methods <p>Caregivers of 173 autistic children (<i>M</i>_<i>age</i> = 2.58 years at data collection) provided information on their children’s sleep (CSHQ; written descriptions) and behaviour (BASC-3, VABS-3). Demographic, ADOS-2 and developmental (MSEL) information were also available. Using parents’ written descriptions and normative sleep data, children were categorized as severe (SSP) or typical (TSP) problems sleepers, or good sleepers (NSP) (Roussis et al., <CitationRef CitationID="CR36">2021</CitationRef>). Kruskal-Wallis, correlation and regression analyses examined sleep and behavioural relationships among these three sleep groups.</p> Results <p>Most children (71.9%) had two or more sleep problems. The TSP and NSP groups did not differ on behaviour, showing significantly less hyperactivity, aggression, attention, and atypicality than the SSP group. Night waking/parasomnias, daytime alertness, and sleep initiation/duration for both sleep problem groups strongly correlated with increased hyperactivity, attention, anxiety, depression, and aggression. Sleep explained 38.4% of variance and 61.8% variance in behaviour, and behaviour explained 22.4% of variance and 32.1% of variance in sleep, for the TSP and SSP groups respectively.</p> Conclusion <p>Reciprocal relationships between sleep and behaviour in autism emphasise the importance of addressing sleep problems in young autistic children, at the time of diagnosis, as they can negatively impact behaviour and well-being.</p>