To become overtired means to exhaust oneself physically or mentally by exerting too much effort, working for an extended period of time without rest or taking on too much work. Full definition
We always want to base bedtime off the amount of time that a child can tolerate being awake before becoming overtired which will be different depending on their age. (familysleepinstitute.com)
This often backfires because Baby becomes overtired, and chronically sleep - deprived. (diaperpincorner.com)
When you keep a flexible schedule, you can help to ensure that your twins don't become overtired, and you can start to get ahead of sleep. (twiniversity.com)