Eight hours. That's what we're told. That's what we aim for. And yet every morning we feel as if someone had woken us in the middle of something important. What's happening here is neither chance nor weakness — it's the physics of the circadian rhythm, something most of us know almost nothing about.
How to read the numbers: Target = the value we aim for · Risk = what to watch out for · Fact = a finding from research or biology
Recommended range
Expert bodies recommend 7–9 hours of sleep for adults, and consistently shorter sleep is linked to poorer immunity, metabolism and mood. A regular bedtime and wake-up time matter too – including at weekends.
One sleep cycle
Sleep runs in cycles of light, deep and REM stages lasting roughly 90 minutes, repeating several times a night. If you wake up groggy, try shifting your alarm so you are not woken in the middle of deep sleep.
With <6h of sleep
Studies show that people who sleep less than 6 hours have a weaker immune response and are more prone to infections; the percentage is indicative and varies between studies. Pay particular attention to getting enough sleep before vaccinations and during cold season.
According to research, more than 60% of adults in Central Europe sleep with significant disruption of their sleep architecture — meaning they wake up in the wrong phases, spend too little time in deep sleep, or shift their rhythm at weekends so much that their body thinks it has crossed two time zones.
Hours in bed ≠ quality sleep
Sleep isn't one continuous state. It's a cycle that repeats 4 to 6 times a night. Each cycle lasts around 90 minutes and contains several phases — from light sleep through deep sleep to REM (rapid eye movement). Each phase has its own biological role, and when one is interrupted or cut short, the body remembers.
Deep sleep (stage N3) is when the body physically regenerates — repairing tissue, producing growth hormone, clearing metabolic waste from cells. REM sleep is when the brain processes information, consolidates memory and regulates emotions. Wake someone from deep sleep and they'll be disoriented for minutes. Interrupt their REM and they'll be emotionally unstable for the whole morning.
The circadian rhythm — the clockmaker we ignore
Every cell in your body has its own clock. Not as a metaphor — biological clocks, genes that switch on and off in a 24-hour cycle. These clocks are governed by one thing: light. Specifically, blue light, which enters through the eyes and tells the brain where we are in the 24-hour cycle.
The problem with modern life is simple: the light we get in the evening from screens, phones and LED lamps has almost the same spectrum as the morning sun. The brain reads it as a "it's morning" signal and delays the production of melatonin — the hormone that prepares the body for sleep. The result? We fall asleep later, our sleep cycles shift, and the alarm wakes us in the wrong phase.
This isn't theory — it's a measurable phenomenon. Cortisol (the wakefulness hormone) should peak high in the morning and be low in the evening. Melatonin should be high in the evening and low in the morning. In most people who use screens late into the night, these curves are shifted by 1 to 3 hours.
Studies show that shifting the circadian rhythm by two hours (e.g. sleeping from 1:00 to 9:00 instead of 23:00 to 7:00) reduces sleep quality parameters by 20–40%, even when the duration stays the same. The body doesn't sleep by the clock — it sleeps by light and darkness.
Weekend jet lag — the phenomenon nobody talks about
It's called social jet lag, and it's one of the most widespread and least discussed causes of poor sleep. On workdays you get up at six. At weekends, at nine. That difference — three hours — is biologically equivalent to flying from Bratislava to Dubai. Every single week.
Research by Till Roenneberg, a chronobiologist from Munich, shows that more than 30% of the population suffers from chronic social jet lag. It shows up as morning fatigue, increased sugar cravings, lowered immunity and poorer concentration in the first half of the working week.
Why most advice doesn't work
The internet is full of thousands of articles on sleep hygiene. Switch off screens an hour before bed. Keep a consistent bedtime. Cool down your room. None of this is bad advice — but it's generic advice for the average person with an average chronotype.
The problem is that people differ chronobiologically. There are four basic chronotypes — from the extreme early bird to the pronounced night owl. An early bird following the "correct" sleep protocol designed for the average person may, paradoxically, sleep worse than if they followed their own natural rhythm.
And that's just one factor. Sleep is affected by cortisol (and therefore your stress levels during the day), magnesium (commonly deficient), body temperature, meal timing, alcohol (which dramatically reduces time in REM, even though it helps you fall asleep), and also by genetic variants of genes such as PERIOD1, PERIOD2 or CLOCK, which determine your individual rhythm.
Where to look for answers
If you want to understand why your sleep isn't working, you need to look at several variables at once. Your chronotype (not what you'd like it to be, but what your body really needs). Your cortisol levels (morning, evening). Melatonin. Magnesium. Your thyroid — an underactive thyroid dramatically lowers sleep quality, yet it is rarely checked. Sleep apnoea — present in a far larger share of people than is commonly thought, including people who don't snore.
Specific protocols for improving sleep do exist — and they work. But how they're set up depends on what exactly your problem is. That's why this article doesn't end with a recipe. It ends with a question: do you know what specifically is disrupting your sleep?
