The figure of eight hours is repeated as though it were a specification. It is the middle of a distribution, and where you sit in that distribution is largely determined by genetics.
The actual range
Most adults function best on something between seven and nine hours. A minority genuinely need less and a minority genuinely need more. People who believe they thrive on five hours are usually wrong, and a small number are not.
How to work out yours
The reliable test is what happens when nothing wakes you. Over a period without an alarm – a holiday works – sleep duration settles at your natural requirement after the first few nights of catching up.
Why regularity matters
The body runs on a circadian rhythm that expects consistency. Going to bed and waking at the same times produces better outcomes than the same total hours taken irregularly.
Shifting your schedule substantially at weekends produces something like the effect of jet lag every week.
What debt does and does not do
Sleeping longer after a short night restores how you feel. Measured markers – metabolic, cognitive, immune – do not fully recover. The debt is partly permanent.
What helps
Consistent timing. Morning light, which sets the clock. A cool, dark room. Caffeine limited to earlier in the day, since its half-life is around five hours.
And getting out of bed when you cannot sleep rather than lying there, which is a core part of the behavioural treatment for insomnia.
When to seek help
Persistent difficulty sleeping is treatable, and the behavioural treatment outperforms medication for long-term outcomes. It is worth asking a doctor about by name.