How many times have you heard this one?
"There's no such thing as overtraining! Only under eating and under recovering!"
This quote is factually incorrect. First because it misinterprets what "overtraining" is and second because it ignores the limits of natural human physiology.
Overtraining is the name of a syndrome, NOT the act of training too much. I've written extensively about this here: What Overtraining Is and Isn't.
It's a physiological state caused by an excess accumulation of physiological, psychological, emotional, environmental, and chemical stress that leads to a sustained decrease in physical and mental performance, and that requires a relatively long recovery period.
Basically your body is under more stress than it can handle – more than it can recover from and adapt to. So the quote about overtraining and under recovering is correct but it's also erroneous. The simple fact is, overtraining is a lack of recovery.
Here's Where This Saying Falls Short
It will lead people to believe that you can compensate for an excessive training volume simply by eating and sleeping more. There are several problems with that belief:
Digestion and absorption of nutrients can become an issue.
Eating 5, 6, or 8 thousand calories per day will overload the digestive system and cause issues. It's also impractical. Unless you're eating mostly junk food, getting in 5-6 thousand calories per day is quite a chore.
It's doable for a few days of course. But try sustaining it for a long time and it's hell. I once tried to eat 6000 calories from "clean" foods and I was constantly cooking and eating, not to mention feeling bloated and gassy.
Your body has a limited capacity to use nutrients to repair and grow muscle.
This is especially true for protein. Utilizing the ingested protein to build muscle is dependent on the rate of protein synthesis, which is itself highly dependent on the level of anabolic hormones in your body: testosterone, growth hormone, IGF-1, and insulin.
At first, increasing protein intake will increase the rate of muscle growth. There are benefits to increasing protein intake up to maybe 1.2 grams per pound of bodyweight (for the natural lifter), but beyond that it stops having any added benefits for muscle growth. The rate of protein synthesis just can't keep up.
Calories can also come from carbs and fats.
Like protein, there's a similar limitation with carbs. And a muscular man of more than 190 pounds can store around 350-500 grams of carbs in his muscle in the form of glycogen. Once glycogen stores are full, the extra carbs will be stored as body fat.
Now, if you keep carbs high every day, you'll never totally deplete muscle glycogen. With a grueling workout and your normal daily activities you'll use maybe 350-400 grams of carbs per day. This means if you ingest more than 400 or maybe 500 grams of carbs per day it will most likely be converted to fat.
And I'm being generous. In most people the limit is likely closer to 300-350. And 400 grams of carbs is 1600 calories. Let's assume you'd have a 250 gram protein intake and you're up to a daily caloric intake of 2600 calories, which is well short of the huge amounts we talked about earlier.
Fat is different than protein and carbs because your body can store an unlimited amount of it.
I like to see fat as a caloric "filler." Once you've consumed the limit of protein you can use to build muscle, and the quantity of carbs that can be stored in the muscle and used for your daily activities, use fat to reach the proper caloric intake level for your goal. If you need 3500 calories per day and you're consuming 2600 calories from carbs and protein, that leaves you 900 calories from fat (100 grams).
While consuming a slight calorie surplus will help you build muscle, consuming too big of a surplus will not speed up your muscle gains.
This is due to the limitation imposed by your natural level of anabolic hormones. It will however, contribute to making you gain fat. The natural lifter's body has a limited capacity to use nutrients for building muscle. Overconsumption won't allow you to be excessive in your training.
What About Sleep?
Getting your 8-10 hours per day will allow you to progress faster and handle more volume than shorter durations. But exceeding that amount won't provide added benefits. And even if it did, who can afford to sleep 14-16 hours per day?