Why does 5-6 hours of sleep feel better than 7-8?

Feeling rested is not the same as sleeping long enough

A short night can feel fine, especially if sleep was uninterrupted and you woke at a natural point in your cycle. That experience is real, but it describes how you felt that morning, not what your sleep is doing over months. In a large study of men aged 40–69, sleep duration itself was not linked to prostate cancer risk across the range studied, while trouble falling or staying asleep was linked to higher risk [1]. The authors read this as a hint that sleep quality, rather than the number of hours, may be the more important consideration [1].

That distinction matters for the 5–6 versus 7–8 question. If someone sleeps 5–6 hours but falls asleep easily and stays asleep, they may be getting better-quality rest than someone who allots 8 hours but wakes repeatedly. The evidence supports quality as a separate dimension worth tracking [1][4], but it does not show that short, high-quality sleep beats longer, high-quality sleep.

When researchers compare the two groups, shorter sleepers often look worse

In a study of adults aged 25–44 in Novosibirsk, men sleeping 5–6 hours had higher systolic blood pressure, higher mean hemodynamic blood pressure, and a higher heart rate than men sleeping 7–8 hours [2]. The same pattern of higher systolic pressure with 5–6 hours appeared in both sexes [2]. These are measured physical differences, not subjective ratings, and they point the opposite way from “shorter feels better.”

Mood tells a similar story over the long run. In a ten-year follow-up of English adults aged 50 and over, sleeping less than 6 hours predicted more depressive symptoms later, compared with 7–8 hours, even after accounting for other factors [4]. Sleep disturbance was also linked to poorer quality of life and lower life satisfaction [4]. So the short-sleep group did not come out ahead on wellbeing.

Why the morning feeling can point the wrong way

One plausible explanation is that people judge sleep by how alert they feel on waking, which depends heavily on whether sleep was broken and on when they woke relative to their body clock. A 5–6 hour night that ends cleanly can leave someone feeling sharper than a restless 8 hours. The prostate cancer study is consistent with this idea, since poor sleep continuity tracked with risk while duration did not [1].

Lifestyle may also blur the picture. In the ageing study, alcohol use and physical inactivity explained only a small part of the link between disturbed sleep and poor quality of life, roughly 8% and 5% respectively [4]. That suggests the sleep–wellbeing connection is not simply a stand-in for other habits, though the authors describe these as partial and limited contributors [4].

What the comparisons do not settle

None of these studies directly tested whether 5–6 hours feels better than 7–8 hours for the same person. They compare groups of people who happen to sleep different amounts, which means other differences between those groups could be doing some of the work. The Novosibirsk findings were statistically significant in men but not in women [2], so the blood-pressure pattern is not uniform across sexes.

There is also a practical caution about very short sleep. In a study of late pregnancy, women who averaged less than 6 hours per night were far more likely to have a particular delivery type, after controlling for infant birth weight [5]. That is a different population and outcome, but it is a reminder that “under 6 hours” is not a safe universal target. The honest answer to why 5–6 can feel better than 7–8 is that the feeling is about sleep quality and timing, while the health comparisons generally favor the longer, uninterrupted night [1][2][4].

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