In a long-term study, toddlers who slept less than 10 hours a night or woke frequently at night tended to have more emotional and behavioural problems at age 5.
The researchers had expected to see a link between sleep and emotional and behavioural problems but were surprised "that the risks were so strong and consistent", said lead author Borge Sivertsen of Uni Research Health and the Norwegian Institute of Public Health in Bergen.
"While only an experimental study can determine causality, our study does suggest that there is an increased risk of developing such problems, also after accounting for a range of other possible factors," Sivertsen told Reuters Health.
The new results come from an ongoing study of 32,662 pairs of mothers and children in Norway supported by the Norwegian Ministry of Health.
The mothers filled out questionnaires when they were 17 weeks pregnant, when the child was 18 months old and again when the child was 5 years old.