The seven to nine hour range is repeated so often that it has become background noise, a number with no way to verify it against one's own life. Whether this number specifically applies to you is another question, and research offers a more precise answer than just counting hours on a tracker.
The number is a range for good reason
Seven to nine hours is the range supported by large-scale research from the National Sleep Foundation and comparable institutions internationally. However, individual needs truly vary within this range, and a small percentage of the population carries documented gene variants associated with good functioning at the lower end. The honest starting point isn't to find the one "right" number and force yourself into it, but to observe where your own body reliably lands when nothing interferes.
The clearest signal: how you function without an alarm clock
One of the most reliable self-tests sleep researchers refer to: on several consecutive days, when your schedule allows, wake up without an alarm clock. If you reliably wake up on your own within a fairly narrow time window and feel truly awake within thirty minutes, that duration is likely close to your actual need. If you need an alarm clock every single day and still feel groggy long after waking up, that's a signal to take seriously instead of ignoring it.
The daytime test that matters more than the tracker
Sleep researchers consistently point to daytime functioning as a more reliable indicator than logged hours. True, excessive daytime sleepiness, fighting off sleep in low-stimulation moments like a quiet meeting or a monotonous drive, is a stronger signal of insufficient sleep than any number on a display. This distinction matters because duration and depth are not the same: someone who logs eight hours with fragmented architecture can feel just as impaired as someone with six hours of solid sleep, and only the daytime experience makes the difference visible.
Why fatigue might not be due to 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 every cell needs to convert food into usable energy, steadily declines from the mid-20s. The same eight hours therefore provide measurably less recovery than before, regardless of sleep quality itself. This is the reasoning behind NMN in the morning, as direct support for this cellular energy system, instead of assuming that more time in bed can compensate for a declining production process that time in bed cannot fix.
The mechanical cause hidden 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 short, unremembered arousal responses that fragment deep and REM sleep without ever fully waking the sleeper. Such a night can log a completely normal duration on the tracker and still lack the depth these hours should provide. This is exactly the gap the nasal strip aims to close: open nasal breathing throughout the night, so that logged hours are hours that count.
A simple way to truly test it
For one week, roughly and without perfectionism, observe two things: how many hours you reliably sleep when no early alarm forces you, and how truly awake you feel in the morning, without covering it up with caffeine. If both values look reasonable and you still feel sluggish, the answer probably isn't more hours, but what happens with the hours you already get.
The honest answer
There's no single number that applies to everyone, but there's a reliable way to find yours: waking up on your own within a consistent window, getting through the day awake without artificial aid, and interpreting persistent fatigue despite sufficient hours as a signal to look at quality and mechanism rather than simply spending more time in bed.