Gist
Why do you feel groggy after waking? This guide explains sleep inertia, how long it can last, naps, caffeine, circadian timing and when to seek advice.
Are sleep cycles really 90 minutes? Learn how sleep architecture actually works, why cycle length varies and what bedtime calculators can and cannot predict.
A practical guide to sleep debt, including a weekly calculation method, worked examples, catch-up sleep, naps, recovery and the limits of sleep-debt maths.