Sleep debt and desynchronized sleep pressure
Sleep runs on two systems — circadian rhythm and sleep pressure that builds like hunger the longer you're awake; sleeping in and irregular timing desync them, and accumulated sleep debt can't simply be repaid.
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Background
Sleep rests on two independent systems. The first is circadian rhythm — the internal clock that sets when you feel sleepy. The second is sleep pressure (the homeostatic process) that builds like hunger: the longer you stay awake, the stronger it gets. You sleep best when the two are in sync — when circadian sleepiness coincides with high sleep pressure.
In ADHD this easily falls apart. Sleeping in shortens waking hours, so not enough pressure builds before the next bedtime — you lie awake and fuel a vicious cycle that pushes the clock ever later. This is a distinct mechanism from delayed phase alone: here the homeostatic side of the equation is sabotaged by irregular wake times and naps.
Accumulated sleep loss can’t be “repaid” with one long weekend lie-in — sleep debt leaves a lasting metabolic and cognitive mark. In a Current Biology (2019) study, sleeping ~5 h/night for a week harmed healthy adults (after-dinner snacking, ~1.4 kg weight gain, lower insulin sensitivity), and weekend catch-up sleep did not undo it. In children with ADHD, cutting sleep by ~1 hour (≈55 min) for 6 nights (Gruber, 2011) pushed attention from the subclinical into the clinical range. A stable, consistent wake time is the lever that re-syncs both systems.