The Key Metric No One Talks About With Sleep

Pete Ross

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If you’ve read any of my work when it comes to health and athletic performance, you’ve probably cottoned on to the fact that I’m a bit of a metrics junkie. I’m not someone who is obsessed with metrics just for the sake of it though — I love metrics because they show me trends and patterns, which I can then experiment with to get better results.

Sleep is something I’ve been collecting a lot of data on over the past 9 months thanks to my Whoop tracker. So much of what I thought I knew was barely scratching the surface. And in the last couple of weeks, I’ve realised that even with all the mainstream health advice on sleep — how to sleep more, how to sleep better, how much sleep to get, no one has talked about what I think might be the most important part of a good night sleep.

REM and Slow Wave Sleep

REM (rapid eye movement, ie dreaming) sleep gets all the buzz and it’s totally important. The thing is, I’ve found that I always get plenty of REM sleep, it’s a non-issue. I usually get a lot of slow wave sleep (the cycle where you physically recover) as well. Yet sometimes I still wake up feeling tired and struggling. So what gives?

Sleep Resting Heart Rate

Regardless of REM or SWS, the one metric that I can rely on for knowing that I’ve slept really well is my average resting heart rate (RHR) during sleep. If I’m anywhere between 43–47 beats per minute (bpm), I wake up feeling like superman. If I’m hitting 50 or over, that’s where things start to decline.

Bear in mind that while the 7 bpm difference above seems small, in the context of my personal heart rate, it’s actually a ~15% variance.

Is it Correlation, or Causation?

The big question for anyone with a scientific background or mindset. I actually couldn’t turn up much research on RHR and quality of sleep. From my experience and observation, it seems to be a cycle of causality. If your RHR gets down to a low average, then you’re making the most of your sleep cycles, which will cause your RHR to drop further.

That cycle also applies in the opposite direction. When your RHR is elevated due to outside factors, it disrupts your sleep cycles meaning you don’t sleep as well, which means your average RHR remains higher. As a result, you’re not as well recovered the next day.

Why does this metric matter?

When we normally think of RHR, we’re thinking of when we’re sitting on the couch watching TV. That would be your awake RHR. Your sleep RHR is actually much lower than your awake RHR, because your body is effectively shutting itself down to rest and repair. During REM sleep, your body is actually paralysed to prevent you injuring yourself while you dream.

With your body at absolute rest, you’re getting the maximum amount of rejuvenation out of your sleep cycles.

The other reason it’s important is because of its specificity. If you’re aiming to get your average RHR as low as possible during sleep, that’s a single, specific metric you’re working towards. And when you’re that specific on a metric, you can get really specific on what behaviours are going to help that metric and what behaviours are going to hinder that metric.

So now instead of “do x to sleep better,” which can feel kind of vague and “where’s the proof?” you’re asking “what do I need to do to get my heart rate down before sleeping?”

So let’s dive in and look at some very commonly cited things we’re told not to do and see how they stack up.

Alcohol before bed

Alcohol is a depressant, so we assume a nightcap is going to help us sleep well. Hell, I was always sceptical of being told not to drink before bed because I always thought I slept really well after having a couple. Research shows that in healthy adults, alcohol does actually help you get to sleep faster, but that’s where the benefits end. That’s because when you have alcohol in your bloodstream, your liver is working to metabolise it and your body produces cortisol. That means your heart rate is going to remain around 10% higher depending on how much you’ve had to drink and how long your body needs to process it.

If I happen to be at a dinner party or having a couple before bed, my average RHR doesn’t get below 52bpm. That’s actually a 20% difference, and the reason I don’t drink within a couple of hours of bed if I want to perform the next day.

If you’re really loaded, to the point that your body is metabolising the alcohol most of the night, then your heart is going to much more elevated the entire time and you’ll wake up feeling like garbage.

The data shows that alcohol consumption prior to sleep has a particularly strong impact on our heart while we sleep.
Notably, the data shows that consuming alcohol prior to sleep is associated with a 10% increase in Average Sleeping Heart Rate. Those who reported consuming alcohol prior to sleep had an Average Sleeping Heart Rate that was nearly 7 beats per minute faster than those who didn’t. That means that when we choose to booze, we may be forcing our heart to beat an extra 3,000 times during the night.

Caffeine

Ok, this one is a no-brainer. We know caffeine is a stimulant. The average half life of caffeine in a healthy adult is around 5 hours. So if you have a coffee at 5pm, that doesn’t mean it’s out of your system by 10pm, it means that only half of it is gone. Depending on how quickly you metabolise coffee, that half life could actually be up to 9 hours.

That changes things somewhat, no? If you consider the fact that even for people with a tolerance for caffeine that you’re going to have an increased heart rate, you want all of it out of your system before you go to bed at night. That means instead of the average advice of steering clear of coffee a few hours before bed, you want to be cutting it off between midday and 2pm.

Eating

There are two reasons not to eat within 3 hours of going to bed. The first is that you’re allowing your body to fast for around 12 hours between meals. The second, which is applicable here, is that when food is digesting, your body’s metabolism going to work, which obviously means that your heart rate is going to be slightly elevated. Digestion is a long and slow process that takes many hours, but the bulk of the blood flow and work happens immediately upon ingestion while the food is being broken down in the stomach.

Gastric emptying generally takes around 2 hours, which means that if you’re sleeping at 10pm, you want to be finishing your dinner by 7pm to ensure your stomach is clear.

Otherwise, if you go to bed with a very full stomach, during the first few hours of sleep your body is still active in trying to break down that food and your heart rate is elevated.

Exercise

A number of people exercise close to bed time, maybe because of weird working hours or maybe because it’s the only time they can get due to their current life circumstances. Obviously it’s not a good idea. Exercise increases your heart rate more than anything and depending on the intensity and duration, your heart rate can stay elevated for a long time afterwards. A really hard workout — be it cardio or weights at the gym shouldn’t finish any closer to your bedtime than 4 hours or so.

This, along with your post workout meal immediately after, will allow your body the time it needs to come back to its rested state and recover properly during sleep. If your heart rate is still elevated when you go to bed, well now your sleep is going to be compromised, when sleep is what allows your body to recover and get stronger.

The worst combination would be getting in a tough workout at 7pm, getting home to eat dinner at around 8:30 and then trying to sleep at 10pm. Your heart rate is going to be elevated well past the time you close your eyes and you’ll feel pretty average the next day.

Unresolved issues

This can be a big one for a lot of people. You’re either in conflict with a family member or friend, or maybe you’ve got too much on at work or some big assignment due. You’ve got a bunch of unresolved feelings or thoughts on it, so instead of being able to sleep your mind is running a hundred miles a minute, causing you to toss and turn. What happens every time you turn over?

Your heart rate spikes, because your whole body is moving.

If your issue is really bad, you might even be producing cortisol, causing your heart rate to increase overall as a result. That’s why it’s a good idea not to let arguments or conflicts fester through to bed time. If you can, try to talk it out before you go to bed so it’s off your mind. Even if it’s not fully resolved, you’ll feel at peace because you’ve at least moved the needle on it.

Sleeping with others

This can be a spouse, a child or a pet. If you’re in a bed together and too close, their movements during the night will disrupt your sleep. I found that when our daughter came into our bed it didn’t matter how long I slept at night, it was poor quality because she would constantly stir next to me. Looking at the sleep log on my Whoop tracker, it was constant spikes in my heart rate. I solved that issue by sleeping in the bed on the floor next to my daughter — hey, it’s not the best solution and I only get to sleep in bed with my wife once a week as a result, but at least it means everyone is well rested.

Solving this could mean doing any number of things:

  • Getting a bigger bed so there’s plenty of room
  • Getting separate sheets/blankets
  • Giving your pet their own sleeping space

So now that we've been throught all of that, what can we do to get our heart rate down before bed?

Nightly routine

A lot of self-improvement articles talk about morning and evening routines. An evening routine is incredibly useful because it promotes the behaviours you need to get your heart rate down. That means when your head hits the pillow, sleep comes more quickly because your heart rate is already at its lowest while you’re awake. What does your nightly routine need to include?

  • No food for 3 hours before bed
  • Exercise (especially strenuous) finished 4 hours before bed
  • Last caffeine consumption 7–9 hours before bed
  • Alcohol out of your system an hour before bed
  • Do a brain dump into a journal of what is causing you stress

It goes without saying that in the last half an hour before bed, you should relax. That means it’s not time to start clearing up all the kids toys, cleaning the kitchen, playing that video game that gets you annoyed, or getting all your things ready for the morning. That stuff should all be done earlier in the evening. The last half an hour should be relaxing with a book, watching the TV or chilling out in whatever fashion suits.

Final thought

Now that you have this information, you can probably pick out a couple of big ones in the list above where you’re pretty far off the mark, so start there and see how it improves for you. That said, don’t get too neurotic over all of this. Having one drink before you go to bed isn’t going to mean you’ll feel wiped out the next day, nor will having a cup of coffee an hour later than you should.

As with everything like this remember, if you’re doing most things right most of the time, then you’re winning. Don’t let the perfect be the enemy of the good.

*I'm not a doctor and none of this should be taken as medical advice. These are merely my own observations and hypotheses that might be worth a try.

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I write about career, performance, psychology, self development and business humour. I'm an author, former national competitor in judo and strongman and a former military instructor.

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