How does a bad night’s sleep affect your workout and results?

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We all know sleep is important. When you get enough, you feel like nothing in the world can stop you. You’re productive, feel great, and the world is like a clear, beautiful picture! But when you don’t sleep enough, it’s like being in your own, personal, foggy-visioned Hell. Everything is irritating, you can hardly focus, and the next person to say you need more sleep is getting smacked. So if your sleep – or lack thereof – can affect your mood and mental performance, can it also affect your physical performance? How about the progress you make during a workout?

The answers are “yes” and a resounding “Hell yes”, respectively!

What does sleep do?

You’ve probably heard that we don’t really understand why humans sleep or really what it does for us. The interesting part is, after thousands of years of scientific development and progress, this is still true! While we know some of the effects that sleep – or rather lack thereof – has on the body, the mechanism behind it is still not really understood. If you’ve ever gone a day or three without sleep, you know exactly how much your body wants to do it. It’s on par with eating and drinking: if you’re not sleeping, you’re gonna have a bad time!

We can spend time comparing sleepiness to hunger, and that’s a great analogy that describes the feelings and physical sensations we have when we’re low on calories or sleep. The problem is, we can look at physiological processes and say “that’s why we need to eat!” but we can’t yet look at similar processes and account for our need to sleep. What we do know is that when humans don’t sleep for long periods of time, they die. Clearly, sleep is a big factor in our health. On the surface we know that sleep helps us think more clearly, gives us down time to repair tissue damage, helps elevate our mood, and generally makes our body chug along more efficiently than when we’re tired.

So, why do we need it? This is what we really don’t know, but science has given us some theories to work with. Note that I’m not saying these are correct, but they’re where we’ve gotten so far.

Inactivity Theory

One of the earlier theories, also sometimes called the adaptive or evolutionary theory. This one looks at sleep as an adaptation to make us less active at night, which helped ancient man survive during a time of the day where other predators were at a distinct advantage over us. Since animals that didn’t go wandering around in the dark were less likely to fall and die, or get eaten by a tiger, they passed on the genes that regulated sleep while the night wandering animals all got eaten.

The simple counter argument is that if this were true, then it would make more sense for the animals who were inactive to also remain awake during the night hours. This way, they aren’t walking around BUT they’re also able to react to danger more quickly. If you wonder what the difference is, then have your cat pounce on your face in the middle of the night and try to react in a life saving way.

Pro-Tip: The cat will win.

Energy Conservation Theory

Back in the day, it wasn’t quite so easy to get food. A major factor in the survival of any animal, including humans, was their ability to compete for and effectively utilize energy resources. Based on this, the energy conservation theory suggests that the primary function of sleep is to reduce an individual’s energy demand and expenditure during part of the day or night, especially at times when it is least efficient to search for food.

There is actually a fair bit of science that supports this one. Studies show that energy metabolism in humans drops by as much as 10% while sleeping, with both body temperature and and calorie needs being reduced.

This theory is often considered related to, or part of, the inactivity theory.

Restorative Theories

This is an interesting theory that has a large body of empirical evidence associated with it, though direct cause-effect evidence is a little scant. The belief is that when you sleep, your body and mind are “restored” in some way. Both human and animal studies show that sleep does have this effect. Most strikingly, animals deprived of sleep saw a total shut-down of their immune systems and then died in a matter of weeks. This fits with happens to humans in the same conditions.

This is further supported by findings that many of the major restorative functions in the body like muscle growth, tissue repair, protein synthesis, and growth hormone release occur mostly, or in some cases only, during sleep.

The functions of the brain are improved with sleep as well. One neat bit of science shows that while we’re awake, neurons in the brain produce adenosine as a by-product of cellular activity. Researchers think that a build up of this substance is partially responsible for the feeling of being tired, and since we know chemical changes in the brain can rapidly and severely alter feelings and perceptions, this seems like a solid theory. To further support this, the feeling of sleepiness can be counteracted by using caffeine, which blocks the actions (and consequences) of adenosine in the brain.

When we’re awake, the brain is firing off those neurons and building up adenosie. During sleep, the body has a chance to clear adenosine from the system, and, as a result, we feel more alert when we wake.

Brain Plasticity Theory

This is one of the newer theories out there with some compelling evidence behind it. Research is finding that sleep patterns are correlated to changes in the structure and organization of the brain. Known as brain plasticity, this is being linked to sleep, and specifically REM sleep, in humans. Infants and children, for example, spend more time sleeping than adults, and more of their time is spent in REM sleep (dreaming sleep) than we spend in the same state as adults.

This body of research has been expanded to show that sleep deprivation has negative impacts on not just active function (i.e. paying attention and speaking coherently) but also on the person’s ability to learn new information. So not only does sleep deprivation make you dumber, it actively prevents you from getting smarter!

How does sleep affect your performance in workouts?

We hit on a lot of factors above where your sleep patterns can help (or hurt) you, and many of those also affect how well you can push through a workout.


Your brain is simply not as on point without sleep as it is with it. Your clarity, both in a strict visual sense and in a “what the Hell am I doing on the bench press” sense, is severely impacted by sleep deprivation. Your neurons can’t fire as effectively, your brain can’t process new information very quickly, and you start to become duller than a box of rocks.

During your workout this could lead to skipped sets, extra reps, mismanagement of weights, and confusion within more complicated workout routines. The biggest issue with most of these things is really Safety. When you aren’t paying attention to your weights or those around you because you just can’t focus, you create a potential hazard for everyone in the area. When your training routine is altered because you keep forgetting what you’re doing, you run the risk of over-training or not actually meeting your training goals for the day.

Emotional Investment

Ever have one of those moments when you haven’t slept and something/one annoyed the crap out of you and you nearly punched them in the head? How about the opposite situation: where you know you should care about what’s going on but you just can’t because you’re too tired? Sleep affects your ability to monitor and respond appropriately to things that trigger an emotional response. The stronger the response, often the worse our sleep-deprived reaction.

If you’re having trouble dampening your emotions (i.e. you’re easily annoyed or angered) then you may find yourself throwing weights, getting annoyed at other gym goers, or getting mad at yourself when things don’t go just right. This could mean you get kicked out of the gym that day, have some seriously negative encounters with others, or maybe just leave disappointed in yourself for failing to perform well (regardless of how you actually did).

If, instead, you’re having trouble getting motivated because you’re too tired to care, you’re likely going to stop well short of your best – or even acceptable – levels of effort during the workout. This could translate to missing reps or sets, giving up part way through your workout, or being careless with your movements.

Physical Limits

When you’re not sleeping, your body isn’t functioning at 100%. Your hormones can quickly get out of sorts, with some being released more and others not being released at all. Poorly managed hormones mean you’re gonna have a bad time.

During the actual workout you may notice that your lifts are lower, or that weights that normally feel easy now feel borderline impossible. You may feel sluggish while running or doing other cardio, and your limbs may feel too far away from your body.

Coordination, too, is part brain and part body. We know that the brain isn’t sending signals very well to itself, which means it’s safe to assume it won’t be talking to your arms and legs too well. This will make movements that require heavy skill – like Double Unders or Barbell Snatches – much more difficult and possibly unsafe to perform.

That’s how not sleeping can affect your during your workout. At this point you many be wondering how lack of sleep affects you after your workout. Funny you should ask…

How does sleep affect your training progress?

To put it simply: not sleeping is almost as bad as not training at all.

One thing many theories of why sleep is important have in common is the thought that sleep allows your body and mind time to renew in some way, shape, or form. When you sleep, your body has the chance to clear out and repair broken down tissue, remove excess waste products from your system (like adenosine, above), and rebuild energy stores. When you get enough sleep, your body has the time to recover, repair, and get ready for the next session. In contrast, when you’re not sleeping your body can’t go from fatigued back up to baseline or higher. Since the ultimate goal of training is to improve, not being able to even get back to the start line is not a good thing.

Here are some recommendations:

  • Try to get 7+ hours of sleep per night. More than 10’ish doesn’t seem to help all that much, so 10 hours is the ideal (which I wish I got!).
  • Don’t skip significant amounts of sleep to get your workout in. If you wake up 30 min earlier but are still getting 7+ hours sleep per night, then you’re probably fine. If you’re cutting you sleep from barely 8 hours down to 5-6 just to get to the gym more often, it’s time to tweak your schedule.
  • Remember that downtime during the day is the perfect time for a nap if you’re not sleeping much at night. Giving yourself an hour to sleep during lunch can help you make massive gains in getting rid of that sleep debt.
  • Workout when it works for you, as long as you’re getting at least 36 hours of rest between sessions that hit the same muscle groups.
  • Early morning workouts are not better than late night, nor is the reverse true. It matters that you train when you’re able to train effectively and safely. Everything else is just details.

At the end of the day, sleep is more useful and important than many other things you do with your time. If you’re not getting enough, then you need to make a change. Stop wandering around Facebook at 11:00 PM, put your phone out of reach so you can’t fiddle with it, and turn off the lights. Start setting yourself up to have a good sleep environment, and the sleep will follow.