Rest days are an important part of your transformation. Give yourself at least one rest day every week to allow your body to repair and help prevent injury. The more you push your body to handle more strenuous physical activity, the more you need a rest day.
Why You Need Rest Days
Aside from just needing a mental break from pushing yourself in the gym, if you're doing something strenuous like weightlifting or running, you're causing micro-tears to your muscles. Your body needs a rest day to repair, rebuild and come back stronger.
Even if you're not doing a high-impact workout, if you never rest, you won't see any progress and you'll risk over-training, which comes with some nasty side effects like trouble sleeping, reduced motivation, constant muscle soreness and depression.
However, don’t embrace your former couch-potato side and translate "rest day" as a full day of lying around the house. Consider your rest day as a day without the challenge of strenuous exercise — not a day without any physical activity at all.
You want the benefit of circulation to deliver nutrients to your muscles, and movement enhances circulation. Lounging on the couch for 12 hours does not (sorry!).