Matthew Walker's lab at UC Berkeley has shown that a single night of disrupted sleep increases next-day anxiety by up to 30%, and that the deep, non-REM phase of sleep is the brain's overnight 'anxiolytic' — the time the prefrontal cortex re-establishes regulatory control over the amygdala. [1]
That's why a hard day after a bad night feels disproportionately heavy: your emotional braking system is running on a flat battery. It isn't weakness. It's neurochemistry.
The most evidence-based sleep advice is also the most boring. Keep a consistent wake time, even on weekends. Get morning light within an hour of waking. Stop caffeine by early afternoon. Keep the bedroom cool and dim. None of this is a hack; it's just respecting what the system needs. [2]
If you're sleeping badly for more than a few weeks, CBT-I (cognitive behavioural therapy for insomnia) outperforms sleeping pills in the long term and is recommended as the first-line treatment by the American College of Physicians.