I know rest days seem counter intuitive. You can't exactly make progress if you don't do any work, but the fact is you are doing work. In fact, you are doing more work now than ever. That's why your body 100% needs recovery days to help you get in shape.
How Your Muscles Heal
When you workout, you are literally damaging your muscles. Microscopic tears form on the muscle, but don't worry! You want this damage to happen.
Imagine a shirt that's been torn on the side because you got too big to fit. It's still a good shirt, it does what it's supposed to, but it needs to be fixed up. In order to keep the shirt while giving you the extra space you need to not rip it again, an extra piece of fabric is sewn underneath the tear. Now the shirt is a bit larger and can still be worn comfortably.
This is what happens to your muscle tears.The protein you eat is sewn into the muscle and a little extra is added because the body assumes (correctly) you'll be pushing around that same amount of weight again. The more muscle extra muscle on your arm, the easier the reps will be, explaining why you need to keep adding weight every time.
Your Muscles Need Rest
So when does this sewing happen? If you guessed when you're sleeping, you are absolutely correct. By drifting into a state of blissful unconsciousness, our bodies get to work repairing everything, including your muscles. Now, this healing can't be done all in one night. That's why you need an additional day of rest. By giving your body ample time to repair everything correctly, you're ensuring proper muscle growth for your next round.
When You Don't Rest
Overtraining syndrome (OTS) is a real thing that can totally mess you up. While it definitely takes a lot of pressure to reach the more extreme levels, it is nevertheless a danger for people that are all about working as hard as they can for as long as they can. Don't think you can overexert yourself without consequences. Train smart, and you will make healthy gains.
A Note on Routines
Rest days are main reason why finding and sticking to a trusted routine is so important, especially as a beginner. There are far too many routines touted by novices and magazines that will mess you up. Be smart. Find a program with trusted, healthy results.
Co-host of HeroesForge, founder of Battle to Be Better, gamer, singer and all around happy person.