Toddlers who miss a nap are unhappy, stressed, anxious, less excited by positive events, and less interested in their world – factors which may lead to future psychological problems if sleep deprivation continues. The study, published in the Journal of Sleep Research, involved toddlers who had missed a nap and were then filmed working on puzzles. Those who were tired were less happy when they completed a puzzle, and more stressed when they were unable to finish a puzzle. Tired toddlers were also less curious about solving a difficult puzzle than well rested children, a problem that may lead them to be less likely to ask for help and to develop poor coping skills.