The seven-to-nine-hour range is repeated so often that it has become background noise, a number with no way of verifying it against your own life. Whether this number actually applies to you is another question, and research offers a more precise answer for that than counting hours on a tracker.
The number is a range for a good reason
Seven to nine hours is the range supported internationally by large-scale research from the National Sleep Foundation and comparable institutions. However, individual needs really do vary within this range, and a small portion of the population carries documented gene variants associated with functioning well at the lower end. The honest starting point is not to find the one "correct" number and force yourself into it, but to observe where your own body reliably lands when nothing gets in the way.
The clearest signal: how you function without an alarm
One of the most reliable self-tests that sleep researchers point to is waking up without an alarm on several consecutive days when the calendar allows. If you reliably wake up on your own within a fairly narrow window and feel truly awake within thirty minutes, this duration is likely close to your actual need. If you need an alarm every single day and feel groggy long after waking up, that is a signal to take seriously instead of ignoring it.
The daytime test that counts for more than a tracker
Sleep researchers consistently point to daytime functioning as a more reliable indicator than logged hours. True, excessive daytime sleepiness—fighting against sleep in low-stimulation moments like a quiet meeting or a monotonous car ride—is a stronger signal of insufficient sleep than any number on a display. This distinction matters because duration and depth are not the same thing: someone who logs eight hours with fragmented architecture can feel just as impaired as someone with six hours of solid sleep, and only the experience during the day makes the difference visible.
Why your fatigue might not be about the hours at all
If you reliably get seven to nine hours and still wake up tired, the more useful question shifts from "how much" to "how well." Two variables explain a large part of this discrepancy. The first is cellular: NAD+, the coenzyme that every cell needs to convert food into usable energy, steadily declines from your mid-20s onwards. Therefore, the same eight hours provide measurably less recovery than before, regardless of sleep quality itself. This is the rationale behind taking NMN in the morning as direct support for this cellular energy system, rather than assuming that more time in bed can compensate for a declining production process that time in bed cannot repair.
The mechanical cause hiding in a normal-looking night
The second variable is the airways. Restricted nasal breathing during sleep, even a mild restriction without audible snoring, reduces nocturnal oxygen and triggers brief, unremembered arousals that fragment deep sleep and REM without ever fully waking the sleeper. Such a night can log a perfectly normal duration on a tracker and still fall short of the depth those hours should provide. This is exactly the gap the nasal strip is intended to bridge: open nasal breathing through the night so that the logged hours are hours that count.
A simple way to really check it
For a week, observe two things, roughly and without perfectionism: how many hours you sleep reliably when no early alarm forces you up, and how awake you honestly feel in the morning without masking it with caffeine. If both values look reasonable and you still drag yourself around, the answer likely isn't more hours, but rather what is happening with the hours you are already getting.
The honest answer
There is no number that applies to everyone, but there is a reliable way to find yours: wake up on your own within a consistent window, get through the day without artificial help, and read persistent fatigue despite sufficient hours as a signal to look at quality and mechanics instead of simply spending more time in bed.