How important is sleep for fitness and muscle growth? Nothing is more important. In fact, lack of adequate sleep and rest is probably why you're not progressing faster than you are.
Sleep and fitness go hand in hand. But how much sleep do you need? I need at least 8 hours per night. But yesterday I was feeling tired, more so than usual. So yesterday I took a 2 hour nap (and woke up feeling completely refreshed) and then slept for 9 hours last night. Whether we realize it or not, there is a strong correlation between sleep and muscle growth. So when it comes to maximizing muscle recovery and repair, find out how much sleep YOU need as an individual. The goal is to rest enough each night so that you're waking up every day feeling fully refreshed and are able to execute your workouts and other daily tasks with full concentration and motivation. I did that last night, and as a result I was able to get in a solid weight training session in today ...
... after which I ran for 5 miles without any discomfort or feeling of tiredness.
Listening to your body is all about guaranteeing your fitness and health in the long term. If you need to, dial it back, as I did yesterday.