Follow @raisinghealthysleepers for more baby/toddler sleep tips You may have heard of the terms “overtired” and “undertired” before. These play a crucial role in the sleep success of your bub. However, these are not always the cause of sleep problems. Overtiredness significantly contributes to the difficulty of falling asleep. When a child becomes overtired, their cortisol levels tend to increase, leading to a greater resistance to sleep. Overtiredness can look a bit different at different ages.