What is Rest?
This is a pretty silly question at first glance, but there's quite a bit of nuance. We'll start by defining why we need rest in the first place, and you'll see why it gets so complicated!
Fatigue
Fatigue can be system wide (the whole body) or localized to specific muscles. When the whole body experiences fatigue, we tend to refer to it as "Central Nervous System (CNS) Fatigue" and when it is localized to muscles we call it "Peripheral Nervous System (PNS) Fatigue". The CNS is essentially your brain and spinal cord, and when your body is working very hard it costs a LOT of energy for the body to keep moving. When the PNS is fatigued, but the CNS is not, recovery is relatively quick. Muscles receive quite a bit of blood flow (or they should when you stay moving).
Think of fatigue in terms of a bank account. Let's say you get $1000 to spend every month on "Stress". A workout costs more or less depending on the intensity, so lets say each workout costs 25 for a light intensity day, 50 for a moderate intensity (think a good hard workout with some moderate weight), and 75 for high intensity (a heavy strength day and an intense metabolic conditioning). Let's say you get 2 moderate intensity workouts, and one each of light/high intensity workouts each week. That's a total of $200 per week, or $800 for the whole month!
Now include the stress on your body and mind from work, or school, relationships, kids, other finances, medical health, etc or anything that influences your stress levels. That probably costs AT LEAST as much as the workouts themselves for many people. If we add those two up, that's a total of $1600 spent and we only have $1000. That's not even including the general toll it takes to be social or have our own hobbies!
That's where rest comes in!
Rest in this case is your extra income. There are different types, but the first we should talk about it plain and simple: SLEEP. Sleep is a complex behavior composed of high levels of brain activity and low levels of body activity. Sleep is most effective for CNS Fatigue as described above, since it primarily affects the brain and spinal cord.
Specifically, sleep has two functions:
1) Processing information from the day, organizing and sifting through memories, and storing any skilled/practice work for future use.
2) Clearing out CNS waste from accumulated fatigue.
Item #2 is the primary focus here. There's a system called the "Glymphatic system" in our brain that only activates during deep sleep. It is a unique aspect of our physiology that is essentially a rinse-cycle for the brain. Literally! The brain is made of billions of cells which are all accumulating waste from processing throughout the day and at the end of the day the body literally squeezes the brain like a dirty sponge, rinses it with CNS fluid, over and over and over. This is the REASON we have a sleep cycle in the first place. Yes, our brain also processes information during this time too, but the brain needs to remove waste before it can efficiently process information. So, rinse, lather, repeat.
So good, restful sleep is when we get more efficient rinses! Now back to our financial analogy:
Think of 1 sleep cycle as being worth about $10 each cycle. Sleep experts say most people should be shooting for 7-8 hours of sleep, which is about 4 cycles (everyone's sleep cycles vary just a tad). So each night you sleep, you SHOULD be banking about $40! That adds up to $1200 of good-ol rest. Which takes our total "Spending" allowance per month up to $2200, instead of $1000. The total amount of stress we are accumulating leaves us with about $600 to spare! We can then utilize that extra $600 to address other things, such as a particularly stressful day, harder workouts, social interactions, and hobbies!
Okay, what about PNS Fatigue?
PNS Fatigue is unique, because sleep takes care of CNS fatigue primarily, and to a certain degree helps with PNS fatigue too. However, our muscles don't receive much blood flow during sleep, which is why waking up stiff is such a drag. Sometimes getting out of bed when you are sore is particularly rough, but once you get moving its not so bad (sometimes). This is where the idea of "Active Rest" comes from! Active rest is a way to offset some of the PNS fatigue you accumulate from hard workouts by creating blood flow. This can be done with a simple walk, some light or brisk cardio (Zone 2 for you endurance nerds!). Having a day where you ensure that you are getting some generalized, light activity can really benefit your whole body, but in the context of rest it is absolutely important.
So, the next time you are super sore, the last thing you'll want to do is go for a walk or use an arm bike or stationary bike. BUT, its the one thing your body wants the most!
Wrapping it all up
There's SO much more to talk about here, but here's the basics:
Sleep accounts for your ability to handle stress of ALL kinds. Its your mental and physical paycheck!
Active rest accounts for your ability to prevent sore or tight muscles from staying sore or tight for too long!
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