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It happens somewhere between 5 and 6 p.m. You pick up your toddler from daycare, or you're in the middle of making dinner, and without any particular trigger the child dissolves — inconsolable crying, furious reaction to something trivial, complete inability to be redirected. The morning was fine. Nothing went obviously wrong today. But here, in this specific window of time, the child is unreachable.
Parents reach for "tired" as an explanation. The intuition is correct. What it doesn't capture is why tiredness in a toddler expresses itself as this specific kind of dysregulation — why soothing fails, why the reaction is so disproportionate, why dinner sometimes acts like a reset button. The physiology behind evening meltdowns is worth understanding.
This is also not the same thing as colic. Colic is the unexplained intense crying that peaks around 3–4 months of age (see article 16). The evening meltdowns of one- and two-year-olds are a different, physiologically explicable phenomenon.