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 (ha, get it) 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 it’s a good idea to get 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. — A caveat to this: recent research has shown that you can split volume up over the week in a way that does not require that much rest in between sessions. A general working suggestion now is to train the muscles again once they are no longer sore. As a beginner, however, less is often more, and three days per week is often more than adequate.
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 while creating a stimulus, it repairs, you break it down while creating a stimulus again, it repairs some more. If you don’t give your body time to repair, you’re just left with breaking it down endlessly without ever properly adapting to the stimulus which can actually result in muscle loss. Rest is important!
Sustainability is also something to keep in mind. As a coach and trainer, nearly every single person who has come to me has had a story about “trying to get in shape” that entailed something along the lines of working out two hours a day, five to six days a week. I have done that successfully… after building up to it over the course of ~2 years. I also love being in the gym. Most people do not share that passion and have not built up that capacity / do not need to be in the gym that often. It is so important to give yourself a realistic time table and schedule, otherwise you set yourself up for failure.
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, or eventually six times a week, find something that is tried and true and works for you. Building muscle is a long term goal, so treat it like one. And please, get some sleep.