Does caffeine affect HRV?

Probably not if you're a regular drinker. A systematic review of 13 studies found habitual coffee drinkers showed no significant HRV changes from caffeine. In non-habitual drinkers, caffeine slightly increased parasympathetic HRV — the opposite of what most people assume. Tolerance develops quickly. Your morning coffee almost certainly isn't affecting your readings.

Why is my HRV so different from my partner's?

Possibly genetics — twin studies show 47-74% of resting HRV is heritable. But it could also be lifestyle differences: sleep, exercise levels, alcohol intake, medications like beta-blockers or antidepressants, or hormonal factors like menopause. Two people living in the same house can have very different HRV for entirely explainable reasons. Compare to your own trend, not to each other.

Can HRV predict how long I'll live?

There's a statistical relationship at the population level. A meta-analysis of over 38,000 people found those with the lowest HRV had 56% higher all-cause mortality. But this is statistics, not individual prediction. Your HRV reflects your current health — it doesn't determine your future. The things that improve it are the same things that reduce mortality risk independently: exercise, sleep, moderate alcohol.

Do I need to check my HRV every morning?

No. Let your wearable record it in the background and glance at the weekly average occasionally. Daily checking creates anxiety about normal fluctuations. If you're making a lifestyle change and want to see whether it's having an effect, look at weekly trends over a month or two.

What's the difference between RMSSD and SDNN?

Both measure heart rate variability, but in different ways. RMSSD looks at the variation between one heartbeat and the very next one — this specifically captures your recovery system (the parasympathetic nervous system). SDNN looks at the total variation across a longer measurement window, which includes your recovery system, your stress system, and slower background rhythms like breathing and blood pressure cycles. Most wearables (Oura, WHOOP, Garmin, Fitbit) report RMSSD. Apple Watch reports SDNN. The numbers aren't comparable.

The 8-Hour Sleep Myth: How Much Sleep Do You Actually Need?

Q: Is six hours of sleep enough?

For some people, yes. Six hours falls at the lower end of the normal range, and a minority of people genuinely function well on it. The test is how you feel: if you're getting six hours and staying sharp through the day without leaning on caffeine, that's probably your number. If you're getting six and dragging, you're likely under-slept.

Q: Is it better to sleep eight hours straight or split it up?

For most people, one unbroken block fits best with a steady body clock. Split sleep isn't harmful — plenty of people manage it — but a single period is easier to keep consistent. And if you wake briefly in the night and drop back off, that's not broken sleep. That's just sleep.

Q: Can I catch up on sleep at the weekend?

Partly. A lie-in recovers some of the immediate deficit, and you'll feel better for it. But shifting your wake time by two or three hours on a Saturday is exactly the kind of irregularity that unsettles your body clock. A short lie-in or a nap disrupts far less than a long one

Q: Should I stop using my sleep tracker?

Not necessarily. If you glance at the weekly trend, ignore the nightly stage breakdown, and it doesn't make you anxious, it's harmless. But if you check it first thing and it sets the tone for your day, try a couple of weeks without it. A good test: if your watch says you slept badly but you felt fine until you looked, trust how you felt.

A woman waking rested in morning light, having slept well without chasing eight hours.

Key Takeaways

The eight-hour rule isn't science. It came from a factory workers' campaign 200 years ago.
Most adults need somewhere between six and nine hours. There's no single number that's right for everyone.
The simplest way to know if you're getting enough: how you feel the next day.

You've probably been told you need eight hours of sleep. It's one of the most repeated numbers in health.

The eight-hour sleep myth is exactly that — a myth.

Getting a good amount of sleep definitely matters. But the specific number, eight hours, was never a medical finding. And treating it as a target you have to hit has made a lot of people anxious.

I'm an emergency medicine doctor and a CBT-i trained sleep coach, and this is a topic I find genuinely frustrating. So I'd like to make it simple.

In this blog, I'll show you where the eight-hour rule came from, how much sleep you actually need and how to get it.

In this article

Where the 8-Hour Sleep Myth Came From

It came from a cotton mill in Scotland.

In 1817, a mill owner named Robert Owen was campaigning against the working conditions of the time — days of fourteen or sixteen hours. He imagined the day divided into three equal parts, and he had a slogan for it:

Eight hours labour, eight hours recreation, eight hours rest.

The slogan spread, and over the years the "eight hours rest" part slowly turned into a 'health rule'. Nobody had measured how long people actually needed to sleep. There was no science behind it. But the mantra stuck.

So How Much Sleep Do You Actually Need?

Firstly, there is no one-size-fits-all. Individual need depends on many factors including age, health, and genetics. But most adults need somewhere between six and nine hours a night.

We can be a little more precise about where the middle of that range sits. When researchers tracked how long people sleep and how long they live, and plotted one against the other, they got a U-shaped curve.

U-shaped curve showing risk of early death is lowest at around 7 hours of sleep and rises with less or more.

The sweet spot, the number of hours linked to the longest life span, is around 7 hours, not 8.

Now, two things are worth saying about that curve, because it's often misread.

The differences are small, and they're averages. This shows where risk sits across millions of people. It doesn't mean your six-and-a-half-hour night is doing you harm. My own average sits a little under seven hours, and I don't worry about it — and in the next section I'll explain why.

And, similarly, people who regularly need a lot of sleep aren't being harmed by the hours — the long sleep is typically a sign of something else going on underneath, like an undiagnosed illness or poor quality sleep that leaves them needing more. You probably don't need nine or ten hours. If you find you do, and you still wake up tired, that's worth a conversation with your doctor.

So the science gives you a range. But it can't tell you your number — whether you're a six-hour person or an eight-hour one. For that, you need something simpler.

The Simplest Way to Know You’re Getting Enough

Here's the one measure that actually works: how you feel the next day.

If you wake up most days feeling reasonably rested, and get through the day without fighting to stay awake, you're getting enough. Whatever the number was. That's what sleep is for, and it's the one signal that reflects your own body rather than a population average.

It sounds almost too simple, so let me explain why it beats the alternatives. As we've seen, sleep need varies from person to person — so no number, and no device, can tell you what yours is.

And you can't count your own hours accurately. When researchers compare what people think they slept against what actually happened, most of us are off — and poor sleepers are the most off of all. People with insomnia slept, on average, about 46 minutes more than they believed. Lying there convinced you've had three hours is not only miserable, it's often wrong.

Your watch isn't the answer either. Trackers are reasonable at telling whether you're asleep or awake, but far less reliable at how much you actually slept — often out by anywhere from fifteen to forty-five minutes a night against clinical equipment.

And here's why that matters. What you believe about your sleep affects how you feel — even when the belief is wrong. In one striking study, people were told how well they'd slept the night before. The feedback was fake, unconnected to their actual sleep. The ones told they'd slept badly, even though they hadn't, performed worse all day. Believing you slept badly can create a bad day on its own.

So, perhaps the best thing you can do is stop trying to calculate how much you slept — and simply notice how you feel.

I also cover this topic on my YouTube channel - https://www.youtube.com/@stressfreelongevity

Does Sleep Need Change as You Get Older?

As you get older, you might notice your sleep getting lighter, or find yourself waking in the night — and it's easy to assume something's going wrong. For most people, it isn't. It's just how sleep changes with age.

You spend a little less time in deep sleep, and you wake more easily and more often. A large review of healthy people — no sleep disorders, just ordinary ageing — found exactly this pattern. It's normal, not a sign of a problem.

What barely changes is how much sleep you actually need. There's a common belief that older adults need much less, and that isn't really true — the requirement drops only slightly. You need roughly the same as you always did. You just tend to get it less easily, because sleep has become lighter and more broken.

None of this means anything's wrong. The same rule applies at every age: if you feel rested and function well through the day, you're getting what you need.

How to Get the Sleep You Need

Here's the reassuring part. Your body is very good at getting the sleep it needs. The problem is we often get in the way of it. The goal isn't to do more. It's more commonly to remove what's blocking you.

For most people, that comes down to three things.

Keep a regular schedule. Going to bed and getting up at roughly the same time — including weekends — is the single most useful thing you can do. Your body has two internal drivers of sleep: your body clock and your homeostatic sleep drive. A regular routine keeps your behaviour lined up with your body clock, so the two work together rather than against each other — making it easier to drift off and stay asleep. And this has a big impact on our health. One large study found that regular sleep timing predicted health outcomes even better than sleep duration did. If you change only one thing, make it this. You can read more about this on my blog on sleep regularity.

Remove the disruptors. A few common things quietly work against you. Caffeine has a half-life of around five hours, so an afternoon coffee is still in your system at midnight, keeping you from settling. Alcohol can help you drift off but then fragments the second half of the night, impacting your sleep quality. And a bedroom that's too warm, too bright, or too noisy keeps nudging you out of deeper sleep without fully waking you.

Deal with a busy mind. Arguably the least appreciated disruptor of all. If you're lying there running through tomorrow's to-do list, that's not really a sleep problem — it's a stress problem. And sleep despises stress. Bedtime should be an oasis of calm. Some things that help: constructive worry time, where you put all your worries, concerns and to-dos on a list earlier in the evening, so they're out of your head and not waiting for you overnight. A proper wind-down routine — soft lighting, no devices, calm activities like reading. And a few techniques you can use in bed to settle body and mind, such as progressive muscle relaxation, 4-7-8 breathing, or cognitive diffusion. I have a blog coming on this in the next few weeks.

Notice what's not on that list. No supplements. No gadgets. No expense at all. For most people, clearing these blocks is enough.

An open book on a bed under warm lamplight, a calm screen-free wind-down.

When to See Your Doctor

Sometimes clearing the blocks isn't enough, and it's worth getting help.

If you've done the sensible things — a regular schedule, cutting the caffeine and alcohol, winding down — and you're still consistently sleeping under six hours or waking up tired day after day, see your doctor.

The first job is to rule things out. Sleep apnoea is common, treatable, and often missed — loud snoring, or someone noticing you stop breathing in the night, is the classic sign. Thyroid problems, anaemia and depression can all show up as unrefreshing sleep too. A conversation and a simple blood test cover most of it.

If those are excluded and sleep is still a genuine struggle, the next step isn't a stronger sleeping tablet. It's CBT-I — cognitive behavioural therapy for insomnia — which every major sleep authority recommends as the first-line treatment for chronic insomnia. It works better than medication over the long term, and unlike medication, the benefit lasts after you stop. You can access it through books, apps, or a trained therapist. I have my own online course here.

The Bottom Line

Eight hours was never a medical fact. It was a factory slogan for human rights that became ingrained in our psyche.

Most adults need somewhere between six and nine hours. Keep a regular schedule, clear away the things that disrupt you, and leave your worries out of the bedroom.

Then judge it by the only measure that matters — how you feel the next day. If you wake up most days reasonably rested and get through without fighting to stay awake, you're getting enough sleep. Whatever the number was.

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Frequently Asked Questions

Is six hours of sleep enough?

Is it better to sleep eight hours straight or split it up?

Can I catch up on sleep at the weekend?

Why do I feel tired even after eight hours?

Because hours in bed aren't the same as good sleep. Fragmented sleep — most commonly from something like sleep apnoea — can leave you exhausted despite a full night, which is why loud snoring or waking unrefreshed is worth taking seriously. But if your sleep itself is sound and you're still tired, the cause often lies outside sleep altogether: stress, low mood, inactivity, or diet. One useful distinction: sleepiness, where you're fighting to stay awake, usually points to a sleep problem; fatigue, low energy without actually nodding off, often points elsewhere. Either way, tiredness that a good night's sleep never seems to fix is worth raising with your doctor.

Should I stop using my sleep tracker?

Sources

Hirshkowitz M et al. (2015). National Sleep Foundation's sleep time duration recommendations. Sleep Health. ScienceDirect
Cappuccio FP et al. (2010). Sleep duration and all-cause mortality: a systematic review and meta-analysis. Sleep. PubMed
Windred DP et al. (2024). Sleep regularity is a stronger predictor of mortality risk than sleep duration. Sleep. Oxford Academic
Ohayon MM et al. (2004). Meta-analysis of quantitative sleep parameters across the human lifespan. Sleep. PubMed
Draganich C & Erdal K (2014). Placebo sleep affects cognitive functioning. Journal of Experimental Psychology: Learning, Memory, and Cognition. PubMed
National Institute on Aging. Sleep and Older Adults. NIA

Medical disclaimer

This content is for informational purposes only and is not intended as medical advice. Always consult your healthcare provider before making changes to your health routine. What works for one person may not work for another — this is a roadmap, not a prescription.

About Dr. Eoghan Colgan

Emergency medicine physician researching what actually works for longevity. I interview world-class experts in health and longevity and test everything personally. Everything I teach is what I'm implementing myself. More about me →

Round image of Dr Eoghan Colgan with purple scrubs and stethoscope