TL;DR – Your “serious” 5 day workout plan is crap. Unless you’re an advanced lifter on HGH, stop. New lifters, start at 2-3x per week with rest days in between, and if you’re getting into that intermediate area, you might be able to sustain a 4x a week program. Five days a week will leave your body no time to grow, and you’ll just be sore with no noticeable improvement.
So, as it says above, if you are trying (or were) to get ripped over summer and thought “Hey, let’s do 5 days a week. I’ll put in serious effort and get totally ripped!” then you’re going to be sorely surprised. Working out takes serious effort both physically and mentally, but if you try to train like a robotic ape with no off switch, you’re going to have very negative consequences.
There’s two key phrases to keep in mind: Rest days between your workout days, and rest times between sets. This is only going to be about the former (for now).
The key to understanding rest days is first understanding one of the building blocks – sleep.
Whether you’re trying to gain muscle or trying to lose weight, sleep is going to be paramount for your success. Lack of sleep cuts your potential fat loss by around 60% and in fact drastically increases your potential lean mass (muscle) loss! In this study (which is citing this study), the group that had insufficient sleep (5.5 hours nightly) lost 1.4kg, but only 0.6kg was fat, the other 0.9kg was muscle. That’s 1.3lbs and 2lbs respectively! This was only over a period of two weeks. The group that got enough sleep 1.4kg of fat (3lbs) and only 0.1kg of muscle (0.2lbs). So your body needs adequate sleep in order to recover nightly at the very least, showing that time off is essential. But do you need more than just nightly rest? Yes.
You need nightly rest in order to recover and rebuild, but you also need at least 48 hours before training the same muscle group again. This study tested multiple days of lower body movements, specifically focusing on quadriceps (your thighs), and showed everything you don’t want it to: reduced functionality, increased pain.
Your days off are meant to be the days when your muscles are relaxing and repairing – this is how building muscle mass functions. You break it down, it repairs, you break it down again, it repairs some more. If you don’t give your body time to repair, you’re just left with breaking it down endlessly, which intuitively has negative effects. Furthermore, repairing aside, your ability to perform is increasingly hindered. So not only are you unable to repair your muscles correctly, you training becomes half-effort and a poor stimulus anyways.
Possible Objections: But my training does different things on different days! It does back/bicep, chest/tricep, legs, and then repeats!
That’s well and good, but chances are your body is still fatigued, and you don’t know what you’re doing. If you’re not squatting or deadlifting, chances are your program is garbage anyways (unless you’re physically unable to and change them for something else). Squats do work your legs, but you’re also using your core as a stabilizer. Deadlifts work your legs, but also your arms, back, and core. Then you have assistance exercises for those days which also target multiple muscles. And to top it all off, these all take a toll on your central nervous system (CNS) which needs time off to recover. While it can certainly take a beating, it needs a break. Chances are, you’re better off working with a 3x or 4x routine based around major compound movements (Bench, Deadlift, Shoulder Press, and Bench Press, detailed here for movements, here for programs to start).
Most of all, if you burn out, your goals are meaningless. You have to take into account what you can sustain and build your methodology around that. Whether that’s 2x a week with full body splits or 4x a week focusing on specific compound movements for each session, find something that is tried and true and works for you. Building muscle is a long term goal, so treat it like one.