The bright first days. What happens inside your child when the excitement fades and the body finally tells the truth.

On Carrer de Provença, outside the school gates this morning, a boy named Pau drags his backpack instead of carrying it. He wore his new sneakers on the first day — now the soles are scuffed at the toe. His mother calls his name twice. He stops, and when she crouches down to him I can smell the sunscreen on his neck, still thick from yesterday.

Co-regulation — the process by which a calm adult nervous system literally entrains a child's aroused one — is what happens when she crouches. Pau's cortisol doesn't drop because of what she says. It drops because her body, close and unhurried, signals to his that the threat level has changed.

Parents read the withdrawal as reluctance. The research — and there is a decade of it from Gunnar, Talge, and colleagues studying salivary cortisol in school-entry cohorts — says the opposite: the fatigue, the short fuse, the sudden clinginess at pick-up are signs of successful adaptation, not failure. The body is paying down a debt it ran up being brave.

The first weeks of a new school year produce a distinctive signal in the children I follow: elevated engagement, high social responsiveness, sleep shortened by anticipation. What strikes me is not the energy itself but what consistently follows it — a quiet drop, usually around day ten or twelve, when mood flattens and small complaints multiply. We used to read this as adjustment. The data suggests something more specific is happening in the brain.

A 2023 randomized controlled trial published in *Frontiers in Neuroscience* assigned adults diagnosed with chronic fatigue syndrome to either a Tai Chi intervention or a waitlist control group and measured resting-state brain activity using fMRI at baseline and follow-up. The scans revealed reduced functional connectivity within the default mode network in fatigued subjects — specifically between the posterior cingulate cortex and medial prefrontal regions — areas central to self-referential processing and sustained attention. Fatigue scores on the Multidimensional Fatigue Inventory dropped significantly in the Tai Chi group alongside measurable network normalization, suggesting the brain's baseline state during fatigue is not simply rest but a reorganized and metabolically costly configuration.

The mechanism is synaptic homeostasis. During sustained high engagement — new faces, new rules, heightened social monitoring — neurons fire with unusual intensity, and synaptic connections strengthen through repeated activation. This potentiation carries a metabolic cost: adenosine accumulates as a byproduct of neural activity, progressively increasing what sleep researchers call homeostatic sleep pressure. The default mode network, which handles the constant background work of self-monitoring and social prediction, is especially vulnerable to this load. When adenosine levels peak, the system reorganizes downward — and the child suddenly seems flat.

Parents see the slump — the quietness at dinner, the resistance to talking, the sudden tears over something minor — and read it as a social signal. The child didn't connect. Something went wrong. The school may not be the right fit. This reading is understandable and almost entirely backwards. The adenosine-driven downregulation is precisely what follows successful high-effort engagement, not failed engagement. A child who shows no fatigue after the first weeks of school has likely not been fully present in them.