Your brain on five hours
As impaired as two nights without sleep — and you're the last to know
“I only need five hours. I’ve always been like that.”
Do you recognise yourself in that sentence? I hear it more than almost anything else in the clinic — and almost always from the people who need the exact opposite. Here’s the uncomfortable part: they mean it. They genuinely feel fine.
That feeling is not proof you’re the exception. That feeling is the problem.
This piece is about the gap between how well you think you function on too little sleep and how well you actually do. That gap is not small, it is measurable, and it is quietly costing you more than almost anything else you worry about. And the real issue isn’t two bad nights. It’s the person who has run on five hours for years, learned to function on it, and now calls that baseline “normal.” Seems fine, no?
No.
It looks like a train ride with your family where your back suddenly locks while you’re lifting your kid onto your shoulders. It looks like a knee that aches after the same workout you’ve done a hundred times — “why now?” It looks like a cut that takes too long to close, a morning where you wake up already exhausted by the sleep you just had. None of it feels like sleep. So you never connect it to sleep. Ever had that feeling and blamed everything except the obvious?
What “a little less” actually does
We imagine sleep loss as a cliff: either you pulled an all-nighter and you’re wrecked, or you slept and you’re fine. The body doesn’t have those two settings. It runs on a dial — and the dial moves much further, on much less deprivation, than anyone wants to believe.
In a 2024 randomized study out of Columbia University, healthy adults who normally slept seven to nine hours had their sleep cut by just 90 minutes — roughly one sleep cycle — for six weeks. Not one all-nighter. One cycle less. The “I’ll catch up on the weekend” reduction that millions of people run as their permanent default. Their working memory and impulse control measurably declined, and recovered only when they slept properly again. Ninety minutes. That’s the distance between lights-out at eleven and answering “just a few more emails” until half past midnight.
Now push it further. The classic work — Van Dongen and colleagues, Sleep, 2003 — held healthy adults to six hours a night for two weeks. By the end, their performance on attention and reaction tasks had fallen to roughly the level of people who’d gone two entire nights without sleep. Read that again. Two weeks of an ordinary, respectable, busy-professional schedule produced the brain of someone who hadn’t slept in 48 hours.
Now put that brain in charge of something. The colour scheme for the next campaign. Whether the product ships. Whether the hire is right. You make that call on a Tuesday afternoon, convinced you’re sharp. We don’t say “sleep on it” for nothing — we say it because some part of us has always known the daytime brain, underslept, is not to be trusted with anything that matters.
The part that should worry you: they had no idea
Here is the finding almost nobody talks about, and the one that should genuinely unsettle you.
In that same research, the sleep-restricted participants rated their own sleepiness as only slightly worse. Their sense of impairment flattened out after a few days — while their actual performance kept falling the entire time. Their self-awareness and their real capacity came completely apart. They felt adjusted. They were not adjusted. They had simply lost the ability to notice.
This is not one odd study. It’s the consistent finding: push sleep restriction over weeks, and people become subjectively unaware of the impairment they’re carrying. The instrument you’d use to catch the problem — your own judgement — is disabled by the problem. It is trying to measure whether your ruler has shrunk, using the ruler.
So when someone tells me “I feel fine on five hours,” I hear almost no information in it. Of course you feel fine. Feeling fine is a symptom here, not a defence.
And “fine” is the whole point. A “fine” decision builds a “fine” company. Your product is either something people can’t stop talking about, or it’s “fine.” That difference is made by people who have to be sharp
— and half of them are quietly operating two nights deep in a deficit they can’t feel. You wouldn’t sign off a contract you’d only skimmed. You do it with your own brain every day.
Why it compounds instead of fixing itself
One rough night is nothing. The body absorbs it. The danger is accumulation — what researchers call sleep debt — because the deficit stacks while your perception of it flatlines.
Short sleep doesn’t stay politely filed under “tired.” Attention lapses stretch. Reaction times slow. Judgement and working memory erode. And it spills past the brain: even brief stretches of restricted sleep impair how healthy young bodies handle blood sugar, and push stress-hormone levels up the following evening. So bad sleep doesn’t just make tomorrow harder — it winds up the stress response that then wrecks the next night. That’s the loop. Less sleep raises cortisol; raised cortisol degrades sleep; and the person spinning inside it describes themselves as “just a bit busy right now.”
(How cortisol runs this entire machine — and why it may be the most important variable of all — is next week.)
By the time it becomes undeniable — the mistake that shouldn’t have happened, the temper you don’t recognise in yourself, the wall you hit at 2 p.m. every single day — it has been building for weeks. You just weren’t equipped to see it.
Why relocation and travel make it worse
I work with people who’ve just moved countries, or who live on planes for work. This is where the gap turns dangerous.
A new environment attacks sleep from every side at once: an unfamiliar room, a shifted light schedule, a different climate, and a brain running low-grade vigilance because it doesn’t yet trust where it woke up. Sleep gets shorter and lighter at the precise moment the person has to be at their sharpest — new team, new systems, first impressions, the decisions that set the tone for an entire assignment.
And because everything is new, there’s no baseline left to measure against. At home you might catch it: “I’m usually sharper than this.” Abroad, in flux, with your self-assessment already blunted by the sleep loss itself, there’s nothing to compare to. The gap between how you’re doing and how you think you’re doing is widest exactly when the stakes are highest. You do your best — but be honest: how good is your best, really, right now?
The point
This is not a plea to panic over every imperfect night. It’s a warning against trusting the one instrument you instinctively reach for — the feeling of being fine — on the one topic where it fails you most. That instrument is least reliable precisely when you’re most depleted.
So stop asking “do I feel okay on my sleep?” You’ll say yes, and the yes is worthless. Ask the harder question: what in your life or your environment is quietly stealing 90 minutes off your night — and who, or what, would catch that cost before you’re the last person in the room to notice it?
Because the body keeps an honest ledger. It just doesn’t send the invoice until later — and by then, you’ve already made the decisions.
Something to think about for you here: how many problems are left if you are so tired that all you can do is thinking about how tired you are?
Sources
Zimmerman, M.E., St-Onge, M.-P., et al. (2024). The effects of insufficient sleep and adequate sleep on cognitive function in healthy adults. Sleep Health / National Sleep Foundation.
Van Dongen, H.P.A., Maislin, G., Mullington, J.M., Dinges, D.F. (2003). The cumulative cost of additional wakefulness. Sleep, 26(2).
Chronic sleep restriction over six weeks in high-performing adults (2021), Sleep (Oxford Academic) — on subjective unawareness of impairment.
Spiegel, K., Leproult, R., Van Cauter, E. (1999). Impact of sleep debt on metabolic and endocrine function. The Lancet.
Leproult, R., et al. (1997). Sleep loss results in an elevation of cortisol levels the next evening. Sleep.
Adriane Hajduk · Licensed Physiotherapist · Nutritionist · Movement Scientist · Founder, LIVUP